High-cleanliness steel and process for producing the same

ABSTRACT

A process for producing a high-cleanliness steel is provided which can produce, without relying upon a high-cost remelting process, steel products having cleanliness high enough to satisfy requirements for properties of mechanical parts used under severer environmental conditions. The production process comprises the steps of: transferring a molten steel produced in an arc melting furnace or a converter to a ladle furnace to refine the molten steel; subjecting the molten steel to circulation-type degassing; and casting the molten steel into an ingot, wherein, in transferring the molten steel to the ladle furnace, a deoxidizer including aluminum and silicon, is added to previously deoxidize the molten steel, that is, to perform tapping deoxidation before refining in the ladle refining furnace.

TECHNICAL FIELD

[0001] The present invention relates to a high-cleanliness steel for useas steels for mechanical parts required to possess fatigue strength,fatigue life, and quietness, particularly, for example, as steels forrolling bearings, steels for constant velocity joints, steels for gears,steels for continuously variable transmission of toroidal type, steelsfor mechanical structures for cold forging, tool steels, and springsteels, and a process for producing the same.

[0002] Steels for use in mechanical parts required to possess fatiguestrength and fatigue life should be high-cleanliness (low content ofnonmetallic inclusions in steels) steels. Conventional productionprocesses of these high-cleanliness steels include: (A) oxidizingrefining of a molten steel in an arc melting furnace or a converter; (B)reduction refining in a ladle furnace (LF); (C) circulation vacuumdegassing in a circulation-type vacuum degassing device (RH) (PHtreatment); (D) casting of steel ingots by continuous casting orconventional ingot casting, and (E) working of steel ingots by pressforging and heat treatment of steel products. In the process (A), scrapis melted by arc heating, or alternatively, a molten steel is introducedinto a converter where oxidizing refining is performed, followed by thetransfer of the molten steel to a ladle furnace. The temperature, atwhich the molten steel is transferred, is generally a high temperatureof about 30° C. above to less than 100° C. above the melting point ofthe steel. In the process (B), a deoxidizer alloy of aluminum,manganese, silicon, etc. is introduced into the ladle furnace, to whichthe molten steel has been transferred, where reduction refining iscarried out by deoxidation and desulfurization with a desulfurizer toregulate the alloying constituents. A generally accepted knowledge issuch that the effect increased with increasing the treatment time. Inthis process, a long time of more than 60 min is adopted, and thetreatment temperature is generally 50° C. above the melting point of thesteel. In the RH treatment in the process (C), vacuum degassing iscarried out in a circulation vacuum degassing tank while circulating themolten steel through the circulation vacuum degassing tank to performdeoxidation and dehydrogenation. In this case, the amount of the moltensteel circulated is about 5 to 6 times the total amount of the moltensteel. In the process (D), the RH treated molten steel is transferred toa tundish where the molten steel is continuously cast into a bloom, abillet, a slab or the like. Alternatively, the molten steel from theladle is poured directly into a steel ingot mold to cast a steel ingot.In the process (E), for example, a bloom, a billet, a slab, or a steelingot is rolled or forged, followed by heat treatment to prepare a steelproduct which is then shipped.

[0003] When steels having a particularly high level of cleanliness arerequired, in the above process, the cast steel ingot is provided as araw material which is then subjected to vacuum remelting or electroslagremelting to prepare such steels.

[0004] In recent years, mechanical parts have become used under more andmore severe conditions. This has lead to more and more severerequirements for properties of steel products, and steel products havinga higher level of cleanliness have been required in the art. Theabove-described conventional production processes (A) to (E), however,are difficult to meet this demand. In order to meet this demand, steelproducts have been produced by the vacuum remelting or the electroslagremelting. These methods, however, pose a problem of significantlyincreased production cost.

[0005] Under these circumstances, the present invention has been made,and it is an object of the present invention to provide steel productshaving a high level of cleanliness without relying upon the remeltingprocess.

DISCLOSURE OF THE INVENTION

[0006] The present inventors have made extensive and intensive studieson the production process of high-cleanliness steels with a view toattaining the above object. As a result, they have found the cleanlinessof steels can be significantly improved by the following processes.

[0007] First Invention

[0008] Means of the present invention for solving the above problems ofthe prior art will be described. In the conventional process using arefining furnace, such as an arc melting furnace or a converter, meltingand oxidizing refining are mainly carried out, for example, in the arcmelting furnace or the converter, and the reduction period (deoxidation)is carried out in ladle refining. On the other hand, the first inventionis directed to a process for producing a high-cleanliness steel,comprising the steps of: transferring a molten steel produced in an arcmelting furnace or a converter to a ladle furnace to refine the moltensteel; degassing the molten steel, preferably performingcirculation-type vacuum degassing; and then casting the molten steelinto an ingot, wherein a deoxidizer including manganese, aluminum, andsilicon (form of alloy of manganese, aluminum, silicon, etc. is notcritical) are added in an amount on a purity basis of not less than 1 kgper ton of the molten steel by previously placing the deoxidizer in theladle furnace, and/or by adding the deoxidizer to the molten steel inthe course of tapping from the arc melting furnace or the converter intothe ladle, and, in some cases, a slag former, such as CaO, issimultaneously added, whereby tapping deoxidation, wherein the moltensteel is pre-deoxidized before reduction refining in a ladle furnace, iscarried out.

[0009] According to a preferred embodiment of the first presentinvention, the molten steel is transferred to the ladle furnace in sucha manner that the tapping temperature of the molten steel is at least100° C. above, preferably at least 120° C. above, more preferably atleast 150° C. above, the melting point of the steel.

[0010] The refining in the ladle refining furnace is carried out for notmore than 60 min, preferably not more than 45 min, more preferably 25 to45 min, and the degassing is carried out for not less than 25 min. Inparticular, in the circulation-type vacuum degassing device, it is ageneral knowledge that satisfactory results can be obtained by bringingthe amount of the molten steel circulated to not less than 5 times thetotal amount of the molten steel. On the other hand, in the presentinvention, in the circulation-type vacuum degassing device, the amountof the molten steel circulated in the degassing is brought to at least 8times, preferably at least 10 times, particularly preferably at least 15times, larger than the total amount of the molten steel.

[0011] The present invention embraces a high-cleanliness steel producedby the above production process.

[0012] According to the present invention, preferably, the content ofoxygen in the steel is not more than 10 ppm. Preferably, when thecontent of carbon in the steel is less than 0.6% by mass, the content ofoxygen in the steel is not more than 8 ppm. Particularly preferably, inthe case of C≧0.6% by mass, the oxygen content is not more than 6 ppm.

[0013] Preferably, in the steel of the present invention, the number ofoxide inclusions having a size of not less than 20 μM as detected bydissolving the steel product in an acid, for example, oxide inclusionshaving an Al₂O₃ content of not less than 50%, is not more than 40,preferably not more than 30, more preferably not more than 20, per 100 gof the steel product.

[0014] In the steel of the present invention, for example, when themaximum inclusion diameter in 100 mm² of the surface of the steelproduct is measured in 30 sites, the predicted value of the maximuminclusion diameter in 30000 mm² as calculated according to statistics ofextreme values is not more than 60 μm, preferably not more than 40 μm,more preferably not more than 25 μm.

[0015] Second Invention

[0016] The second invention will be described. In the conventionalprocess using a refining furnace, such as an arc melting furnace or aconverter, melting and oxidizing refining are mainly carried out, forexample, in the arc melting furnace or the converter, and the reductionperiod (deoxidation) is carried out in ladle refining. On the otherhand, the present invention is directed to a process for producing ahigh-cleanliness steel, comprising the steps of: transferring a moltensteel produced in an arc melting furnace or a converter to a ladle toperform degassing, preferably perform circulation-type vacuum degassing;transferring the degassed molten steel to a ladle furnace to refine themolten steel; and further performing degassing, preferablycirculation-type vacuum degassing in a circulation-type vacuum degassingdevice.

[0017] According to a preferred embodiment of the present invention, themolten steel is transferred to the ladle in such a manner that thetapping temperature of the molten steel is at least 100° C. above,preferably at least 120° C. above, more preferably at least 150° C.above, the melting point of the steel.

[0018] The refining in the ladle furnace is carried out for not morethan 60 min, preferably not more than 45 min, more preferably 25 to 45min, and the degassing is carried out for not less than 25 min. Inparticular, in the circulation-type vacuum degassing device, it is ageneral knowledge that satisfactory results can be obtained by bringingthe amount of the molten steel circulated to not less than 5 times thetotal amount of the molten steel. On the other hand, in the presentinvention, in the circulation-type vacuum degassing device, the amountof the molten steel circulated in the degassing is brought to at least 8times, preferably at least 10 times, particularly preferably at least 15times, larger than the total amount of the molten steel.

[0019] The present invention embraces the high-cleanliness steelproduced by the above production process.

[0020] According to the present invention, preferably, the content ofoxygen in the steel is not more than 10 ppm. Preferably, when thecontent of carbon in the steel is less than 0.6% by mass, the content ofoxygen in the steel is not more than 8 ppm. Particularly preferably, inthe case of C≧0.6% by mass, the oxygen content is not more than 6 ppm.

[0021] Preferably, in the steel of the present invention, the number ofoxide inclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid, for example, oxide inclusionshaving an Al₂O₃ content of not less than 50%, is not more than 40,preferably not more than 30, more preferably not more than 20, per 100 gof the steel product.

[0022] In the steel of the present invention, for example, when themaximum inclusion diameter in 100 mm² of the surface of the steelproduct is measured in 30 sites, the predicted value of the maximuminclusion diameter in 30000 mm² as calculated according to statistics ofextreme values is not more than 60 μm, preferably not more than 40 μm,more preferably not more than 25 μm.

[0023] Third Invention

[0024] The third invention will be described. In the conventionalprocess using a refining furnace, such as an arc melting furnace or aconverter, melting and oxidizing refining are mainly carried out, forexample, in the arc melting furnace or the converter, and the reductionperiod (deoxidation) is carried out in ladle refining furnace. On theother hand, the present invention is directed to a process for producinga high-cleanliness steel, comprising the steps of: subjecting a moltensteel to oxidizing refining in an arc melting furnace or a converter;adding a deoxidizer including manganese, silicon, and aluminum (form ofalloy of manganese, silicon, aluminum, etc. is not critical) in anamount of not less than 2 kg per ton of the molten steel to the moltensteel in the same furnace before tapping to deoxidize the molten steel;transferring the deoxidized molten steel to a ladle furnace to performladle refining; and then circulating the refined molten steel through acirculation-type vacuum degassing device to degas the molten steel.

[0025] According to a preferred embodiment of the present invention, themolten steel is transferred to the ladle furnace in such a manner thatthe tapping temperature of the molten steel is at least 100° C. above,preferably at least 120° C. above, more preferably at least 150° C.above, the melting point of the steel.

[0026] According to the present invention, preferably, the refining inthe ladle furnace is carried out for not more than 60 min, preferablynot more than 45 min, more preferably 25 to 45 min. The degassingsubsequent to this step is generally carried out in a circulation-typevacuum degassing device in such a manner that the amount of the moltensteel circulated is brought to not less than 5 times the total amount ofthe molten steel. On the other hand, in the present invention, in thecirculation-type vacuum degassing device, the amount of the molten steelcirculated in the degassing is brought to at least 8 times, preferablyat least 10 times, particularly preferably at least 15 times, largerthan the total amount of the molten steel, and the degassing time is atleast 25 min.

[0027] The present invention embraces the high-cleanliness steelproduced by the above production process.

[0028] According to the present invention, preferably, the content ofoxygen in the steel is not more than 10 ppm. Preferably, when thecontent of carbon in the steel is less than 0.6% by mass, the content ofoxygen in the steel is not more than 8 ppm. Particularly preferably, inthe case of C a 0.6% by mass, the oxygen content is not more than 6 ppm.

[0029] Preferably, in the steel according to the present invention, thenumber of oxide inclusions having a size of not less than 20 μm asdetected by dissolving the steel product in an acid, for example, oxideinclusions having an Al₂O₃ content of not less than 50%, is not morethan 40, preferably not more than 30, more preferably not more than 20,per 100 g of the steel product.

[0030] In the steel of the present invention, for example, when themaximum inclusion diameter in 100 mm² of the surface of the steelproduct is measured in 30 sites, the predicted value of the maximuminclusion diameter in 30000 mm² as calculated according to statistics ofextreme values is not more than 60 μm, preferably not more than 40 μm,more preferably not more than 25 μm.

[0031] Fourth Invention

[0032] The fourth invention will be described. In the conventionalprocess using a refining furnace, such as an arc melting furnace or aconverter, melting and oxidizing refining are mainly carried out, forexample, in the arc melting furnace or the converter, and the reductionperiod (deoxidation) is carried out in ladle furnace. On the other hand,the present invention is directed to a process for producing ahigh-cleanliness steel, comprising the steps of: transferring a moltensteel produced in an arc melting furnace or a converter to a ladlefurnace to refine the molten steel; subjecting the refined molten steelto circulation-type vacuum degassing; and then casting the degassedmolten steel into an ingot, wherein the refining in the ladle furnace iscarried out for not more than 60 min, preferably not more than 45 min,more preferably 45 to 25 min, and, while the degassing subsequently tothis step is generally carried out for less than 25 min in acirculation-type vacuum degassing device in such a manner that theamount of the molten steel circulated is brought to not less than 5times the total amount of the molten steel, in the present invention, inthe circulation-type vacuum degassing device, the amount of the moltensteel circulated in the degassing is brought to at least 8 times,preferably at least 10 times, particularly preferably at least 15 times,larger than the total amount of the molten steel, and the degassing timeis at least 25 min.

[0033] According to a preferred embodiment of the present invention, themolten steel is transferred to the ladle furnace in such a manner thatthe tapping temperature of the molten steel is at least 100° C. above,preferably at least 120° C. above, more preferably 150° C. above, themelting point of the steel.

[0034] The present invention embraces the high-cleanliness steelproduced by the above production process.

[0035] According to the present invention, preferably, the content ofoxygen in the steel is not more than 10 ppm. Preferably, when thecontent of carbon in the steel is less than 0.6% by mass, the content ofoxygen in the steel is not more than 8 ppm. Particularly preferably, inthe case of C≧0.6% by mass, the oxygen content is not more than 6 ppm.

[0036] Preferably, in the steel according to the present invention, thenumber of oxide inclusions having a size of not less than 20 μm asdetected by dissolving the steel product in an acid, for example, oxideinclusions having an Al₂O₃ content of not less than 50%, is not morethan 40, preferably not more than 30, more preferably not more than 20,per 100 g of the steel product.

[0037] In the steel of the present invention, for example, when themaximum inclusion diameter in 100 mm² of the surface of the steelproduct is measured in 30 sites, the predicted value of the maximuminclusion diameter in 30000 mm² as calculated according to statistics ofextreme values is not more than 60 Mm, preferably not more than 40 μm,more preferably not more than 25 μm.

[0038] Fifth Invention

[0039] The fifth invention will be described. In the conventionalprocess using a refining furnace, such as an arc melting furnace or aconverter, melting and oxidizing refining are mainly carried out, forexample, in the arc melting furnace or the converter, and the reductionperiod (deoxidation) is carried out in ladle refining. On the otherhand, the present invention is directed to a process for producing ahigh-cleanliness steel, comprising the steps of: transferring a moltensteel produced in an arc melting furnace or a converter to a ladle as anout-furnace refining furnace to perform refining; subjecting the moltensteel to circulation-type ladle degassing; and then casting the degassedmolten steel into an ingot, wherein the refining in the ladle is carriedout in such a manner that, in addition to stirring by gas introducedfrom the bottom of the ladle, stirring is carried out by electromagneticinduction, and this ladle refining is carried out for 50 to 80 min,preferably 70 to 80 min.

[0040] According to the present invention, preferably, the ladlerefining by the gas stirring and the electromagnetic stirring in theladle is carried out in an inert atmosphere.

[0041] The present invention embraces the high-cleanliness steelproduced by the above production process.

[0042] According to the present invention, preferably, the content ofoxygen in the steel is not more than 10 ppm. Preferably, when thecontent of carbon in the steel is less than 0.6% by mass, the content ofoxygen in the steel is not more than 8 ppm. Particularly preferably, inthe case of C≧0.6% by mass, the oxygen content is not more than 6 ppm.

[0043] Preferably, in the steel of the present invention, the number ofoxide inclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid, for example, oxide inclusionshaving an Al₂O₃ content of not less than 50%, is not more than 40,preferably not more than 30, more preferably not more than 20, per 100 gof the steel product.

[0044] In the steel of the present invention, for example, when themaximum inclusion diameter in 100 mm² of the surface of the steelproduct is measured in 30 sites, the predicted value of the maximuminclusion diameter in 30000 mm² as calculated according to statistics ofextreme values is not more than 60 μm, preferably not more than 40 μm,more preferably not more than 25 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045]FIG. 1A is a diagram showing the relationship between the use orunuse of tapping deoxidation of steel SUJ 2 and the content of oxygen inproducts, wherein A₁ shows data on the adoption of only tappingdeoxidation according to the present invention defined in claim 1, A₂data on the adoption of tapping deoxidation+high-temperature tappingaccording to the present invention defined in claim 2, A₃ data on theadoption of tapping deoxidation+short-time LF, long-time RH treatmentaccording to the present invention defined in claim 3, A₄ data on theadoption of tapping deoxidation+high-temperature tapping+short-time LF,long-time RH treatment according to the present invention defined inclaim 3, and conventional data on prior art;

[0046]FIG. 1B is a diagram showing the relationship between the use orunuse of tapping deoxidation of steel SCM 435 and the content of oxygenin products, wherein B₁ shows data on the adoption of only tappingdeoxidation according to the present invention defined in claim 1, B₂data on the adoption of tapping deoxidation+high-temperature tappingaccording to the present invention defined in claim 2, B₃ data on theadoption of tapping deoxidation+short-time LF, long-time RH treatmentaccording to the present invention defined in claim 3, B₄ data on theadoption of tapping deoxidation+high-temperature tapping+short-time LF,long-time RH treatment according to the present invention defined inclaim 3, and conventional data on prior art;

[0047]FIG. 1C is a diagram showing the relationship between the use orunuse of tapping deoxidation of steel SUJ 2 and the maximum predictedinclusion diameter, wherein A₁ shows data on the adoption of onlytapping deoxidation according to the present invention defined in claim1, A₂ data on the adoption of tapping deoxidation+high-temperaturetapping according to the present invention defined in claim 2, A₃ dataon the adoption of tapping deoxidation+short-time LF, long-time RHtreatment according to the present invention defined in claim 3, A₄ dataon the adoption of tapping deoxidation+high-temperaturetapping+short-time LF, long-time RH treatment according to the presentinvention defined in claim 3, and conventional data on prior art;

[0048]FIG. 1D is a diagram showing the relationship between the use orunuse of tapping deoxidation of steel SCM 435 and the maximum predictedinclusion diameter, wherein B₁ shows data on the adoption of onlytapping deoxidation according to the present invention defined in claim1, B₂ data on the adoption of tapping deoxidation+high-temperaturetapping according to the present invention defined in claim 2, B₃ dataon the adoption of tapping deoxidation+short-time LF, long-time RHtreatment according to the present invention defined in claim 3, B₄ dataon the adoption of tapping deoxidation+high-temperaturetapping+short-time LF, long-time RH treatment according to the presentinvention defined in claim 3, and conventional data on prior art;

[0049]FIG. 1E is a diagram showing the relationship between the use orunuse of tapping deoxidation of steel SUJ 2 and the L₁₀ life, wherein A₁shows data on the adoption of only tapping deoxidation according to thepresent invention defined in claim 1, A data on the adoption of tappingdeoxidation+high-temperature tapping according to the present inventiondefined in claim 2, A₃ data on the adoption of tappingdeoxidation+short-time LF, long-time RH treatment according to thepresent invention defined in claim 3, A₄ data on the adoption of tappingdeoxidation+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention defined in claim 3, andconventional data on prior art;

[0050]FIG. 1F is a diagram showing the relationship between the use orunuse of tapping deoxidation of steel SCM 435 and the L₁₀ life, whereinB₁ shows data on the adoption of only tapping deoxidation according tothe present invention defined in claim 1, B₂ data on the adoption oftapping deoxidation+high-temperature tapping according to the presentinvention defined in claim 2, B₃ data on the adoption of tappingdeoxidation+short-time LF, long-time RH treatment according to thepresent invention defined in claim 3, B₄ data on the adoption of tappingdeoxidation+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention defined in claim 3, andconventional data on prior art;

[0051]FIG. 2A is a diagram showing the relationship between the use orunuse of W-RH treatment of steel SUJ 2 and the content of oxygen inproducts, wherein A₁ shows data on the adoption of only W-RH treatmentaccording to the present invention, A₂ data on the adoption of W-RHtreatment+high-temperature tapping according to the present invention,A₃ data on the adoption of W-RH treatment+short-time LF, long-time RHtreatment according to the present invention, A₄ data on the adoption ofW-RH treatment+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention, and conventional data onprior art;

[0052]FIG. 2B is a diagram showing the relationship between the use orunuse of W-RH treatment of steel SCM 435 and the content of oxygen inproducts, wherein B₁ shows data on the adoption of only W-RH treatmentaccording to the present invention, B₂ data on the adoption of W-RHtreatment+high-temperature tapping according to the present invention,B₃ data on the adoption of W-RH treatment+short-time LF, long-time RHtreatment according to the present invention, B₄ data on the adoption ofW-RH treatment+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention, and conventional data onprior art;

[0053]FIG. 2C is a diagram showing the relationship between the use orunuse of W-RH treatment of steel SUJ 2 and the maximum predictedinclusion diameter, wherein A₁ shows data on the adoption of only W-RHtreatment according to the present invention, A₂ data on the adoption ofW-RH treatment+high-temperature tapping according to the presentinvention, A₃ data on the adoption of W-RH treatment+short-time LF,long-time RH treatment according to the present invention, A₄ data onthe adoption of W-RH treatment+high-temperature tapping+short-time LF,long-time RH treatment according to the present invention, andconventional data on prior art;

[0054]FIG. 2D is a diagram showing the relationship between the use orunuse of W-RH treatment of steel SCM 435 and the maximum predictedinclusion diameter, wherein B₁ shows data on the adoption of only W-RHtreatment according to the present invention, B₂ data on the adoption ofW-RH treatment+high-temperature tapping according to the presentinvention, B₃ data on the adoption of W-RH treatment+short-time LF,long-time RH treatment according to the present invention, B₄ data onthe adoption of W-RH treatment+high-temperature tapping+short-time LF,long-time RH treatment according to the present invention, andconventional data on prior art;

[0055]FIG. 2E is a diagram showing the relationship between the use orunuse of W-RH treatment of steel SUJ 2 and the L₁₀ life, wherein A,shows data on the adoption of only W-RH treatment according to thepresent invention, A₂ data on the adoption of W-RHtreatment+high-temperature tapping according to the present invention,A₃ data on the adoption of W-RH treatment+short-time LF, long-time RHtreatment according to the present invention, A₄ data on the adoption ofW-RH treatment+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention, and conventional data onprior art;

[0056]FIG. 2F is a diagram showing the relationship between the use orunuse of W-RH treatment of steel SCM 435 and the L₁₀ life, wherein B₁shows data on the adoption of only W-RH treatment according to thepresent invention, B₂ data on the adoption of W-RHtreatment+high-temperature tapping according to the present invention,B₃ data on the adoption of W-RH treatment+short-time LF, long-time RHtreatment according to the present invention, B₄ data on the adoption ofW-RH treatment+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention, and conventional data onprior art;

[0057]FIG. 3A is a diagram showing the oxygen content of products in 10(heats) according to the process of the present invention usingin-furnace deoxidation in the treatment of a molten steel of steel SUJ2, and the oxygen content of products in 10 (heats) according to theconventional process wherein the in-furnace deoxidation is not carriedout;

[0058]FIG. 3B is a diagram showing the oxygen content of products in 10(heats) according to the process of the present invention usingin-furnace deoxidation in the treatment of a molten steel of steel SCM435, and the oxygen content of products in 10 (heats) according to theconventional process wherein the in-furnace deoxidation is not carriedout;

[0059]FIG. 3C is a diagram showing the maximum predicted inclusiondiameter according to statistics of extreme values in products in 10(heats) according to the process of the present invention usingin-furnace deoxidation in the treatment of a molten steel of steel SUJ2, and the maximum predicted inclusion diameter in products in 10(heats) according to the conventional process wherein the in-furnacedeoxidation is not carried out;

[0060]FIG. 3D is a diagram showing the maximum predicted inclusiondiameter according to statistics of extreme values in products in 10(heats) according to the process of the present invention usingin-furnace deoxidation in the treatment of a molten steel of steel SCM435, and the maximum predicted inclusion diameter in products in 10(heats) according to the conventional process wherein the in-furnacedeoxidation is not carried out;

[0061]FIG. 3E is a diagram showing the L₁₀ life as determined by thethrust rolling service life test of products in 10 (heats) according tothe process of the present invention using in-furnace deoxidation in thetreatment of a molten steel of steel SUJ 2, and the L₁₀ life of productsin 10 (heats) according to the conventional process wherein thein-furnace deoxidation is not carried out;

[0062]FIG. 3F is a diagram showing the L₁₀ life as determined by thethrust rolling service life test of products in 10 (heats) according tothe process of the present invention using in-furnace deoxidation in thetreatment of a molten steel of steel SCM 435, and the L₁₀ life ofproducts in 10 (heats) according to the conventional process wherein thein-furnace deoxidation is not carried out;

[0063]FIG. 4A is a diagram showing the oxygen content of products in 10(heats) according to the process of the present invention usingshort-time LF treatment and long-time RH treatment in treatment of amolten steel of steel SUJ 2, and the oxygen content of products in 10(heats) according to the conventional process using long-time LFtreatment and short-time RH treatment;

[0064]FIG. 4B is a diagram showing the oxygen content of products in 10(heats) according to the process of the present invention usingshort-time LF treatment and long-time RH treatment in the treatment of amolten steel of steel SCM 435, and the oxygen content of products in 10(heats) according to the conventional process using long-time LFtreatment and short-time RH treatment;

[0065]FIG. 4C is a diagram showing the maximum predicted inclusiondiameter according to statistics of extreme values in products in 10(heats) according to the process of the present invention usingshort-time LF treatment and long-time RH treatment in treatment of amolten steel of steel SUJ 2, and the maximum predicted inclusiondiameter in products in 10 (heats) according to the conventional processusing long-time LF treatment and short-time RH treatment;

[0066]FIG. 4D is a diagram showing the maximum predicted inclusiondiameter according to statistics of extreme values in products in 10(heats) according to the process of the present invention usingshort-time LF treatment and long-time RH treatment in the treatment of amolten steel of steel SCM 435, and the maximum predicted inclusiondiameter in products in 10 (heats) according to the conventional processusing long-time LF treatment and short-time RH treatment;

[0067]FIG. 4E is a diagram showing the L₁₀ life as determined by thethrust rolling service life test of products in 10 (heats) according tothe process of the present invention using short-time LF treatment andlong-time RH treatment in treatment of a molten steel of steel SUJ 2,and the L₁₀ life of products in 10 (heats) according to the conventionalprocess using long-time LF treatment and short-time RH treatment; and

[0068]FIG. 4F is a diagram showing the L₁₀ life as determined by thethrust rolling service life test of products in 10 (heats) according tothe process of the present invention using short-time LF treatment andlong-time RH treatment in treatment of a molten steel of steel SCM 435,and the L₁₀ life of products in 10 (heats) according to the conventionalprocess using long-time LF treatment and short-time RH treatment.

BEST MODE FOR CRYING OUT THE INVENTION

[0069] First Invention

[0070] A preferred production process of a high-cleanliness steelaccording to the first invention comprises the following steps (1) to(5).

[0071] (1) In the conventional steel production process using a refiningfurnace, such as an arc melting furnace or a converter, melting andoxidizing refining are mainly carried out in the arc melting furnace orthe converter, and the reduction period (deoxidation) is carried out ina ladle refining furnace. On the other hand, according to the presentinvention, a molten steel is subjected to oxidizing refining in an arcmelting furnace or a converter. The molten steel is then brought to apredetermined chemical composition and a predetermined temperature, and,in tapping the molten steel from the melting furnace, a deoxidizerincluding manganese, aluminum, and silicon (form of alloy of manganese,aluminum, silicon, etc. is not critical) is added in an amount on apurity basis of not less than 1 kg per ton of the molten steel bypreviously placing the deoxidizer in the ladle, and/or by adding thedeoxidizer to the molten steel in the course of tapping into the ladle,and, in some cases, a slag former, such as CaO, is simultaneously added.The addition of this deoxidizer is the step which is most important tothe present invention. The addition of the deoxidizer before the ladlerefining, which has hitherto been regarded as unnecessary, to reduce theoxygen content to some extent before the reduction period refining inthe ladle furnace can finally realize the production of steels havinglow oxygen content. The reason for this is as follows. The deoxidation,in a system wherein the dissolved oxygen in the molten steel is presentin a satisfactory amount of not less than 100 ppm, results in theformation of a relatively large deoxidation product which can be easilyfloated and can be separated. As a result, the total content of oxygenin the molten steel can be significantly lowered to not more than 50ppm.

[0072] (2) The pre-deoxidized molten steel is transferred to a ladlefurnace where the molten steel is subjected to reduction refining, andthe chemical composition of the steel is regulated.

[0073] (3) The molten steel, which has been subjected to reductionrefining and regulation of chemical composition, is degassed,particularly is circulated through a circulation-type vacuum degassingdevice to perform degassing, and the chemical composition of the steelis finally regulated.

[0074] (4) The molten steel, which has been degassed and subjected tofinal regulation of the chemical composition, is cast into an ingot.

[0075] (5) The ingot is press forged into a product shape which is thenoptionally heat treated to provide a steel product.

[0076] In the preferred production process of a high-cleanliness steelaccording to the present invention, among the steps (1) to (5), the step(2) of transferring the molten steel to a ladle furnace is carried outin such a manner that, while the molten steel is generally tapped at atemperature of about 50° C. above the melting point of the steel, in thepresent invention, the molten steel is tapped at a temperature of atleast 100° C. above, preferably at least 120° C. above, more preferably150° C. above, the melting point of the steel. By virtue of this, thedeoxidizer added at the time of tapping and the metal and slag in theprevious treatment can be completely dissolved or separated, whereby theseparation and dropping of the metal and slag into the molten steel inan advanced refining state during the ladle refining, thereby increasingthe oxygen content, can be prevented, and, at the same time, in therefining furnace, the initial slag forming property and the reactivitycan be improved. Specifically, the reduced metal deposited in theprevious treatment is oxidized in a period between the previoustreatment and this treatment, and when the metal begins to dissolve inthis reduction period operation, particularly at the end of thereduction period operation, the equilibrium condition is broken. As aresult, the molten steel is partially contaminated. For this reason, thedeposited metal is dissolved in the molten steel being tapped before thereduction, and, this dissolved metal, together with the tapped moltensteel, is deoxidized.

[0077] In the above step, while a refining time longer than 60 min isgenerally regarded as offering a better effect, in the preferredproduction process of a high-cleanliness steel according to the presentinvention, the refining in the ladle refining furnace is carried out fornot more than 60 min, preferably not more than 45 min, more preferably25 to 45 min, and, while it is a general knowledge that a degassing timeof less than 25 min suffices for satisfactory results, the degassing inthe preferred production process of the present invention is carried outfor not less than 25 min. In particular, in the circulation-type vacuumdegassing device, it is a general knowledge that satisfactory resultscan be obtained by bringing the amount of the molten steel circulated toabout 5 times the total amount of the molten steel. On the other hand,in the present invention, in the circulation-type vacuum degassingdevice, the amount of the molten steel circulated in the degassing isbrought to at least 8 times, preferably at least 10 times, morepreferably at least 15 times, larger than the total amount of the moltensteel. By virtue of this constitution, the time of ladle refining,wherein refining is carried out while heating, can be brought to aminimum necessary time, and, in the step of degassing not involvingheating, the floating separation time for oxide inclusions can besatisfactorily ensured. This can prevent an increase in oxygen contentcaused by the contamination from refractories or slag on the inner sideof the ladle furnace, and, at the same time, the formation of largeinclusions having a size of not less than about 20 μm can be prevented.In the circulation-type vacuum degassing, particularly since a nozzle isdipped in the molten steel and only the molten steel is circulated, theslag on the upper surface of the molten steel is in a satisfactorilyquiet state. Therefore, the number of oxide inclusions from slag intothe molten steel is fewer than that during the reduction period processin the ladle refining furnace. Therefore, in the pre-deoxidized moltensteel, the adoption of a satisfactorily long degassing time can realizea significant reduction of even relatively small deoxidation products.

[0078] The present invention embraces a high-cleanliness steel producedby the above means.

[0079] The high-cleanliness steel according to the present invention ispreferably a high-cleanliness steel, excellent particularly in rollingfatigue life, which is characterized in that the content of oxygen inthe steel is not more than 10 ppm; preferably, when the content ofcarbon in the steel is less than 0.6% by mass, the content of oxygen inthe steel is not more than 8 ppm; and, particularly preferably, in thecase of C≧0.6% by mass, the oxygen content is not more than 6 ppm. It isgenerally known that lowering the oxygen content can contribute toimproved rolling fatigue life. Among the steels produced by theproduction process according to the present invention, high-cleanlinesssteels having an oxygen content of not more than 10 ppm, preferably notmore than 8 ppm in the case of C<0.6% by mass in the steel, particularlypreferably not more than 6 ppm in the case of C≧0.6% by mass, stablyexhibit excellent rolling fatigue life.

[0080] Further, the present invention embraces, among the abovehigh-cleanliness steels, high-cleanliness steels possessing excellentrolling fatigue life and fatigue strength, which are characterized inthat the number of oxide inclusions having a size of not less than 20 μmas detected by dissolving the steel product in an acid, for example,oxide inclusions having an Al₂O₃ content of not less than 50%, is notmore than 40, preferably not more than 30, more preferably not more than20, per 100 g of the steel product. This evaluation method for steelproducts reflects both the oxygen content and the maximum inclusiondiameter in a predetermined volume. Regarding the fatigue strength,fatigue life, and quietness, in the case of steels having the sameoxygen content, oxide inclusions having a certain large size areharmful, and, in particular, oxide inclusions having a size of not lessthan 20 μm are harmful. Therefore, among the steels produced by theprocess according to the present invention, steels, wherein the numberof oxide inclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid is not more than 40, preferablynot more than 30, particularly preferably not more than 20, per 100 g ofthe steel product, are high-cleanliness steels having both excellentrolling fatigue life and excellent fatigue strength and, in addition,excellent quietness.

[0081] The high-cleanliness steels according to the present inventionfurther include high-cleanliness steels, which are excellentparticularly in rotating bending fatigue strength and cyclic stressfatigue strength and are characterized in that, when the maximuminclusion diameter in 100 mm² of the cross-section of the steel productis measured in 30 sites, the predicted value of the maximum inclusiondiameter in 30000 mm² as calculated according to statistics of extremevalues is not more than 60 μm, preferably not more than 40 μm, morepreferably not more than 25 μm. The cyclic stress fatigue strength andthe fatigue limit are known to greatly depend upon the maximum inclusiondiameter in a predetermined volume. This is disclosed in Japanese PatentLaid-Open No. 194121/1999 of which the applicant is identical to that inthe application of the present invention. High-cleanliness steels,wherein, for example, typically when the maximum inclusion diameter in100 mm² of the cross-section of the steel product is measured in 30sites, the predicted value of the maximum inclusion diameter in 30000mm² as calculated according to statistics of extreme values is not morethan 60 μm, preferably not more than 40 μm, more preferably not morethan 25 μm, stably exhibit excellent fatigue strength. In this case, thehigh-cleanliness steels have an oxygen content of not more than 10 ppm,preferably not more than 8 ppm in the case of C<0.6% by mass in thesteel, particularly preferably not more than 6 ppm in the case of C≧0.6%by mass, and a predicted value of maximum inclusion diameter of not morethan 60 μm, preferably not more than 40 μm, more preferably not morethan 25 μm. The steels produced by the process according to the presentinvention are high-cleanliness steels possessing both excellent rollingfatigue life and excellent fatigue strength. While acid dissolution is avery time-consuming, troublesome work, the above method, which, withoutsteel product dissolution work, can observe a certain area under amicroscope to statistically predict the maximum inclusion diameter, isadvantageously simple. Further, in particular, regarding fatigue createdby cyclic stress of tensile compression, it is known that the maximumdiameter of inclusions present at a site susceptible to failure is agreat factor which governs the strength. This method, which canstatistically predict this maximum diameter, is advantageous.

[0082] Second Invention

[0083] A preferred production process of a high-cleanliness steelaccording to the second invention comprises the following steps (1) to(6).

[0084] (1) A molten steel is subjected to oxidizing refining in an arcmelting furnace or a converter to prepare a molten steel having apredetermined chemical composition and a predetermined temperature.

[0085] (2) The molten steel is then pre-degassed. Specifically, themolten steel is degassed, for example, by circulating the molten steelthrough a circulation-type vacuum degassing device. This step ofdegassing is most important to the present invention. In general, themolten steel produced in step (1) is directly subjected to reductionrefining in a ladle furnace. By contrast, according to the presentinvention, the molten steel is pre-degassed before the reductionrefining. This pre-degassing can contribute to significantly improvedcleanliness of finally obtained steels.

[0086] (3) The molten steel degassed in step (2) is subjected toreduction refining and regulation of chemical composition in a ladlefurnace.

[0087] (4) The molten steel, which has been subjected to reductionrefining and regulation of chemical composition in step (3), is furtherdegassed by circulating the molten steel through a circulation-typevacuum degassing device, and, in addition, the chemical composition ofthe steel is finally regulated.

[0088] (5) The molten steel, which has been degassed and subjected tofinal regulation of the chemical composition, is cast into an ingot.

[0089] (6) The ingot is press forged into a product shape which is thenoptionally heat treated to provide a steel product.

[0090] In the preferred production process of a high-cleanliness steelaccording to the present invention, in the steps (1) to (6), intransferring the molten steel after step (2) to a ladle furnace for step(3), while the molten steel is generally tapped at a temperature ofabout 50° C. above the melting point of the steel, the molten steel istapped at a temperature of at least 100° C. above, preferably at least120° C. above, more preferably 150° C. above, the melting point of thesteel. In the present specification, tapping at an elevated temperatureis referred to as high-temperature tapping. By virtue of thisconstitution, the deoxidizer added at the time of tapping and the metaland slag in the previous treatment can be completely dissolved orseparated, whereby the separation and dropping of the metal and slaginto the molten steel in an advanced refining state during the ladlerefining, thereby increasing the oxygen content, can be prevented, and,at the same time, in the refining furnace, the initial slag formingproperty and the reactivity can be improved. Specifically, the reducedmetal deposited in the previous treatment is oxidized in a periodbetween the previous treatment and this treatment, and when the metalbegins to dissolve in this reduction period operation, particularly atthe end of the reduction period operation, the equilibrium condition isbroken. As a result, the molten steel is partially contaminated. Forthis reason, the deposited metal is dissolved in the molten steel beingtapped before the reduction, and, this dissolved metal, together withthe tapped molten steel, is deoxidized.

[0091] In the ladle refining in step (3), while a refining time longerthan 60 min is generally regarded as offering a better effect, in thepresent invention, the refining in the ladle furnace in step (3) iscarried out for not more than 60 min, preferably not more than 45 min,more preferably 25 to 45 min, and, regarding degassing after the ladlerefining, while it is a general knowledge that a degassing time of lessthan 25 min suffices for satisfactory results, in the present invention,the degassing in the preferred production process of the presentinvention is carried out for not less than 25 min. In particular, in thecirculation-type vacuum degassing device, it is a general knowledge thatsatisfactory results can be obtained by bringing the amount of themolten steel circulated to about 5 times the total amount of the moltensteel. On the other hand, in the preferred production process, in thecirculation-type vacuum degassing device, the amount of the molten steelcirculated in the degassing is brought to at least 8 times, preferablyat least 10 times, more preferably at least 15 times, larger than thetotal amount of the molten steel. By virtue of this constitution, thetime of ladle refining, wherein refining is carried out while heating,can be brought to a minimum necessary time, and, in the step ofdegassing not involving heating, the floating separation time for oxideinclusions can be satisfactorily ensured. This can prevent an increasein oxygen content caused by the contamination from refractories or slagon the inner side of the ladle furnace, and, at the same time, theformation of large inclusions having a size of not less than about 20 μmcan be prevented. In the circulation-type vacuum degassing, particularlysince a nozzle is dipped in the molten steel and only the molten steelis circulated, the slag on the upper surface of the molten steel is in asatisfactorily quiet state. Therefore, the number of oxide inclusionsfrom slag into the molten steel is fewer than that during the reductionperiod process in the ladle furnace. Therefore, in the pre-deoxidizedmolten steel, the adoption of a satisfactorily long degassing time canrealize a significant reduction of even relatively small deoxidationproducts. In the present specification, this method is called short-timeLF, long-time RH treatment or short LF, long RH treatment.

[0092] The present invention embraces a high-cleanliness steel producedby the above means.

[0093] The high-cleanliness steel according to the present invention ispreferably a high-cleanliness steel, excellent particularly in rollingfatigue life, which is characterized in that the content of oxygen inthe steel is not more than 10 ppm; preferably, when the content ofcarbon in the steel is less than 0.6% by mass, the content of oxygen inthe steel is not more than 8 ppm; and, particularly preferably, in thecase of C≧0.6% by mass, the oxygen content is not more than 6 ppm. It isgenerally known that lowering the oxygen content can contribute toimproved rolling fatigue life. Among the steels produced by theproduction process according to the present invention, high-cleanlinesssteels having an oxygen content of not more than 10 ppm, preferably notmore than 8 ppm in the case of C<0.6% by mass in the steel, particularlypreferably not more than 6 ppm in the case of C≧0.6% by mass, stablyexhibit excellent rolling fatigue life.

[0094] Further, according to a preferred embodiment, the steels producedaccording to the process of the present invention includehigh-cleanliness steels possessing excellent rolling fatigue life andfatigue strength, which are characterized in that the number of oxideinclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid, for example, oxide inclusionshaving an Al₂O₃ content of not less than 50%, is not more than 40,preferably not more than 30, more preferably not more than 20, per 100 gof the steel product. This evaluation method for steel products reflectsboth the oxygen content and the maximum inclusion diameter in apredetermined volume. Regarding the fatigue strength, fatigue life, andquietness, in the case of steels having the same oxygen content, oxideinclusions having a certain large size are harmful, and, in particular,oxide inclusions having a size of not less than 20 μm are harmful.Therefore, among the steels produced by the process according to thepresent invention, steels, wherein the number of oxide inclusions havinga size of not less than 20 μm as detected by dissolving the steelproduct in an acid is not more than 40, preferably not more than 30,particularly preferably not more than 20, per 100 g of the steelproduct, are high-cleanliness steels having both excellent rollingfatigue life and excellent fatigue strength and, in addition, excellentquietness.

[0095] According to a preferred embodiment, the high-cleanliness steelsaccording to the present invention further include high-cleanlinesssteels, which are excellent particularly in rotating bending fatiguestrength and cyclic stress fatigue strength and are characterized inthat, when the maximum inclusion diameter in 100 mm² of thecross-section of the steel product is measured in 30 sites, thepredicted value of the maximum inclusion diameter in 30000 mm² ascalculated according to statistics of extreme values is not more than 60μm, preferably not more than 40 μm, more preferably not more than 25 μm.The cyclic stress fatigue strength and the fatigue limit are known togreatly depend upon the maximum inclusion diameter in a predeterminedvolume. This is disclosed in Japanese Patent Laid-Open No. 194121/1999of which the applicant is identical to that in the application of thepresent invention. High-cleanliness steels, wherein, for example,typically when the maximum inclusion diameter in 100 mm of thecross-section of the steel product is measured in 30 sites, thepredicted value of the maximum inclusion diameter in 30000 mm² ascalculated according to statistics of extreme values is not more than 60μm, preferably not more than 40 μm, more preferably not more than 25 μm,stably exhibit excellent fatigue strength. In this case, thehigh-cleanliness steels have an oxygen content of not more than 10 ppm,preferably not more than 8 ppm in the case of C<0.6% by mass in thesteel, particularly preferably not more than 6 ppm in the case of C≧0.6%by mass, and a predicted value of maximum inclusion diameter of not morethan 60 μm, preferably not more than 40 μm, more preferably not morethan 25 μm. The steels produced by the process according to the presentinvention are high-cleanliness steels possessing both excellent rollingfatigue life and excellent fatigue strength. While acid dissolution is avery time-consuming, troublesome work, the above method, which, withoutsteel product dissolution work, can observe a certain area under amicroscope to statistically predict the maximum inclusion diameter, isadvantageously simple. Further, in particular, regarding fatigue createdby cyclic stress of tensile compression, it is known that the maximumdiameter of inclusions present at a site susceptible to failure is agreat factor which governs the strength. This method, which canstatistically predict this maximum diameter, is advantageous.

[0096] Third Invention

[0097] A preferred production process of a high-cleanliness steelaccording to the third invention comprises the following steps (1) to(5).

[0098] (1) A molten steel is subjected to oxidizing refining in an arcmelting furnace or a converter. Subsequently, in the same furnace, adeoxidizer including manganese, silicon, and aluminum (form of alloy ofmanganese, silicon, and aluminum, etc. is not critical) is added in anamount of not less than 2 kg per ton of the molten metal, and, in somecases, a slag former, such as CaO, is simultaneously added to deoxidizethe molten steel. The deoxidized molten steel is then transferred to aladle. The deoxidation in a steel making furnace, such as an arc meltingfurnace or a converter, is a most important step in the presentinvention. The deoxidation before the ladle refining, which has hithertobeen regarded as unnecessary, to reduce the oxygen content to someextent before the ladle refining can finally realize the production ofsteels having low oxygen content.

[0099] (2) The molten steel transferred to the ladle is subjected toreduction refining and regulation of chemical composition in a ladlerefining furnace.

[0100] (3) The molten steel, which has been subjected to reductionrefining and regulation of chemical composition in step (2), is degassedby circulating the molten steel through a circulation-type vacuumdegassing device, and, in addition, the chemical composition of thesteel is finally regulated.

[0101] (4) The molten steel, which has been degassed and subjected tofinal regulation of the chemical composition in step (3), is cast intoan ingot.

[0102] (5) The ingot is press forged into a product shape which is thenoptionally heat treated to provide a steel product.

[0103] In the preferred production process of a high-cleanliness steelaccording to the present invention, regarding step (1), wherein themolten steel is transferred to the ladle furnace, among the steps (1) to(5), while the molten steel is generally tapped at a temperature ofabout 50° C. above the melting point of the steel, in the presentinvention, the molten steel is transferred at a temperature of at least100° C. above, preferably at least 120° C. above, more preferably 150°C. above, the melting point of the steel. By virtue of thisconstitution, the metal deposited around the ladle can be fullydissolved in the molten steel, and the slag can also be fully floated,whereby the separation and dropping of the metal and slag into themolten steel in an advanced refining state during the ladle refining,thereby increasing the oxygen content, can be prevented.

[0104] According to a preferred embodiment, in the ladle refining in theabove step, while a refining time longer than 60 min is generallyregarded as offering a better effect, in the present invention, therefining in the ladle furnace is carried out for not more than 60 min,preferably not more than 45 min, more preferably 25 to 45 min, and,regarding degassing in step (3), while it is a general knowledge that adegassing time of less than 25 min suffices for satisfactory results,that is, it is a general knowledge that satisfactory results can beobtained by bringing the amount of the molten steel circulated to about5 times the total amount of the molten steel, in the present invention,the amount of the molten steel circulated in the circulation-typedegassing device is brought to at least 8 times, preferably at least 10times, more preferably at least 15 times, larger than the total amountof the molten steel, to perform degassing for a long period of time,i.e., not less than 25 min. By virtue of this constitution, the time ofladle refining, wherein refining is carried out while heating, can bebrought to a minimum necessary time, and, in the step of degassing notinvolving heating, the floating separation time for oxide inclusions canbe satisfactorily ensured. This can prevent an increase in oxygencontent caused by the contamination from refractories or slag on theinner side of the ladle refining furnace, and, at the same time, theformation of large inclusions having a size of not less than about 20 μmcan be prevented. In the circulation-type vacuum degassing, particularlysince a nozzle is dipped in the molten steel and only the molten steelis circulated, the slag on the upper surface of the molten steel is in asatisfactorily quiet state. Therefore, the number of oxide inclusionsfrom slag into the molten steel is fewer than that during the reductionperiod process in the ladle refining furnace. Therefore, in thepre-deoxidized molten steel, the adoption of a satisfactorily longdegassing time can realize a significant reduction of even relativelysmall deoxidation products. In the present specification, this method iscalled short-time LF, long-time RH treatment or short LF, long RHtreatment.

[0105] The present invention embraces a high-cleanliness steel producedby the above means.

[0106] According to a preferred embodiment, the high-cleanliness steelaccording to the present invention is a high-cleanliness steel,excellent particularly in rolling fatigue life, which is characterizedin that the content of oxygen in the steel is not more than 10 ppm;preferably, when the content of carbon in the steel is less than 0.6% bymass, the content of oxygen in the steel is not more than 8 ppm; and,particularly preferably, in the case of C m 0.6% by mass, the oxygencontent is not more than 6 ppm. It is generally known that lowering theoxygen content can contribute to improved rolling fatigue life. Amongthe steels produced by the production process according to the presentinvention, high-cleanliness steels having an oxygen content of not morethan 10 ppm, preferably not more than 8 ppm in the case of C<0.6% bymass in the steel, particularly preferably not more than 6 ppm in thecase of C≧0.6% by mass, stably exhibit excellent rolling fatigue life.

[0107] Further, according to a preferred embodiment, the steels producedaccording to the process of the present invention includehigh-cleanliness steels possessing excellent rolling fatigue life andfatigue strength, which are characterized in that the number of oxideinclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid, for example, oxide inclusionshaving an Al₂O₃ content of not less than 50%, is not more than 40,preferably not more than 30, more preferably not more than 20, per 100 gof the steel product. This evaluation method for steel products reflectsboth the oxygen content and the maximum inclusion diameter in apredetermined volume. Regarding the fatigue strength, fatigue life, andquietness, in the case of steels having the same oxygen content, oxideinclusions having a certain large size are harmful, and, in particular,oxide inclusions having a size of not less than 20 μm are harmful.Therefore, among the steels produced by the process according to thepresent invention, steels, wherein the number of oxide inclusions havinga size of not less than 20 μm (for example, having an Al₂O₃ content ofnot less than 50%) as detected by dissolving the steel product in anacid is not more than 40, preferably not more than 30, particularlypreferably not more than 20, per 100 g of the steel product, arehigh-cleanliness steels having both excellent rolling fatigue life andexcellent fatigue strength and, in addition, excellent quietness.

[0108] According to a preferred embodiment, the high-cleanliness steelsaccording to the present invention further include high-cleanlinesssteels, which are excellent particularly in rotating bending fatiguestrength and cyclic stress fatigue strength and are characterized inthat, when the maximum inclusion diameter in 100 mm² of thecross-section of the steel product is measured in 30 sites, thepredicted value of the maximum inclusion diameter in 30000 mm² ascalculated according to statistics of extreme values is not more than 60μm, preferably not more than 40 μm, more preferably not more than 25 μm.The cyclic stress fatigue strength and the fatigue limit are known togreatly depend upon the maximum inclusion diameter in a predeterminedvolume. This is disclosed in Japanese Patent Laid-Open No. 194121/1999of which the applicant is identical to that in the application of thepresent invention. High-cleanliness steels, wherein, for example,typically when the maximum inclusion diameter in 100 mm² of thecross-section of the steel product is measured in 30 sites, thepredicted value of the maximum inclusion diameter in 30000 mm² ascalculated according to statistics of extreme values is not more than 60μm, preferably not more than 40 μm more preferably not more than 25 μm,stably exhibit excellent fatigue strength. In this case, thehigh-cleanliness steels have an oxygen content of not more than 10 ppm,preferably not more than 8 ppm in the case of C<0.6% by mass in thesteel, particularly preferably not more than 6 ppm in the case of C≧0.6%by mass, and a predicted value of maximum inclusion diameter of not morethan 60 μm, preferably not more than 40 μm, more preferably not morethan 25 μm. The steels produced by the process according to the presentinvention are high-cleanliness steels possessing both excellent rollingfatigue life and excellent fatigue strength. While acid dissolution is avery time-consuming, troublesome work, the above method, which, withoutsteel product dissolution work, can observe a certain area under amicroscope to statistically predict the maximum inclusion diameter, isadvantageously simple. Further, particularly in fatigue created bycyclic stress of tensile compression, it is known that the maximumdiameter of inclusions present at a site susceptible to failure is agreat factor which governs the strength. This method, which canstatistically predict this maximum diameter, is advantageous.

[0109] Fourth Invention

[0110] A preferred production process of a high-cleanliness steelaccording to the fourth invention comprises the following steps (1) to(5).

[0111] (1) A molten steel is subjected to oxidizing refining in an arcmelting furnace or a converter to prepare a molten steel having apredetermined chemical composition and a predetermined temperature whichis then transferred to a ladle furnace.

[0112] (2) The molten steel transferred to the ladle furnace issubjected to reduction refining in a ladle furnace and the chemicalcomposition of the molten steel is regulated. At that time, in the ladlefurnace, it is a general knowledge that an stirring gas is blown throughthe bottom of the ladle at 1.5 to 5.0 N.l/min/t to forcibly agitate themolten steel and, in this case, an stirring time longer than 60 minprovides better effect. On the other hand, in the present invention, therefining time in the ladle refining is brought to not more than 60 min,preferably not more than 45 min, more preferably 25 to 45 min.

[0113] (3) The molten steel, which has been subjected to reductionrefining and regulation of chemical composition in step (2), is degassedby circulating the molten steel through a circulation-type vacuumdegassing device, and, in addition, the chemical composition of thesteel is finally regulated. In this case, it is a general knowledge thatthe degassing time is less than 25 min and, in a circulation-type vacuumdegassing device, satisfactory results are obtained by bringing theamount of the molten steel circulated to about 5 times the total amountof the molten steel. On the other hand, in the present invention, theamount of the molten steel circulated is brought to at least 8 times,preferably at least 10 times, more preferably at least 15 times thetotal amount of the molten steel, and the degassing is carried out for alonger period of time, that is, for not less than 25 min. The steps (2)and (3) are most important to the present invention. The ladle refiningtime for refining while heating in step (2) is brought to a necessaryminimum time, and the degassing not involving heating in step (3),particularly circulation-type vacuum degassing is carried out in such amanner that a nozzle is dipped in the molten steel and only the moltensteel is circulated. Therefore, the slag on the upper surface of themolten steel is in a satisfactorily quiet state, and, thus, the numberof oxide inclusions from slag into the molten steel is fewer than thatduring the reduction period process in the ladle furnace. In thissystem, when the floating separation time for oxide inclusions issatisfactorily ensured, an increase in oxygen content caused bycontamination from refractories or slag on the inner side of the ladlefurnace can be prevented and, in addition, the formation of largeinclusions having a size of not less than about 30 μm can be prevented.This can realize the production of high-cleanliness steels.

[0114] (4) The molten steel, which has been subjected to finalregulation of the chemical composition in step (3), is cast into aningot.

[0115] (5) The ingot is press forged into a product shape which is thenoptionally heat treated to provide a steel product.

[0116] In the production process of a high-cleanliness steel, accordingto a preferred embodiment, in the steps (1) to (5), in transferring themolten steel after step (1) to the ladle refining furnace, while themolten steel is generally tapped at a temperature of about 50° C. abovethe melting point of the steel, in the present invention, the moltensteel is tapped at a temperature of at least 100° C. above, preferablyat least 120° C. above, more preferably 150° C. above, the melting pointof the steel. By virtue of this constitution, the metal deposited aroundthe ladle furnace can be fully dissolved in the molten steel, and theslag can be fully floated, whereby the separation and dropping of themetal and slag into the molten steel in an advanced refining stateduring the ladle refining, thereby increasing the oxygen content, can beprevented.

[0117] The present invention embraces a high-cleanliness steel producedby the above means.

[0118] According to a preferred embodiment, the high-cleanliness steelaccording to the present invention is a high-cleanliness steel,excellent particularly in rolling fatigue life, which is characterizedin that the content of oxygen in the steel is not more than 10 ppm;preferably, when the content of carbon in the steel is less than 0.6% bymass, the content of oxygen in the steel is not more than 8 ppm; and,Particularly preferably, in the case of C≧0.6% by mass, the oxygencontent is not more than 6 ppm. It is generally known that lowering theoxygen content can contribute to improved rolling fatigue life. Amongthe steels produced by the production process according to the presentinvention, high-cleanliness steels having an oxygen content of not morethan 10 ppm, preferably not more than 8 ppm in the case of C<0.6% bymass in the steel, particularly preferably not more than 6 ppm in thecase of C≧0.6% by mass, stably exhibit excellent rolling fatigue life.

[0119] Further, according to a preferred embodiment, the steels producedaccording to the process of the present invention includehigh-cleanliness steels possessing excellent rolling fatigue life andfatigue strength, which are characterized in that the number of oxideinclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid, for example, oxide inclusionshaving an Al₂O₃ content of not less than 50%, is not more than 40,preferably not more than 30, more preferably not more than 20, per 100 gof the steel product. This evaluation method for steel products reflectsboth the oxygen content and the maximum inclusion diameter in apredetermined volume. Regarding the fatigue strength, fatigue life, andquietness, in the case of steels having the same oxygen content, oxideinclusions having a certain large size are harmful, and, in particular,oxide inclusions having a size of not less than 20 μm are harmful.Therefore, among the steels produced by the process according to thepresent invention, steels, wherein the number of oxide inclusions havinga size of not less than 20 PM (for example, having an Al₂O₃ content ofnot less than 50%) as detected by dissolving the steel product in anacid is not more than 40, preferably not more than 30, more preferablynot more than 20, per 100 g of the steel product, are high-cleanlinesssteels having both excellent rolling fatigue life and excellent fatiguestrength and, in addition, excellent quietness.

[0120] According to a preferred embodiment, the steels according to thepresent invention further include high-cleanliness steels, which areexcellent particularly in rotating bending fatigue strength and cyclicstress fatigue strength and are characterized in that, when the maximuminclusion diameter in 100 mm² of the cross-section of the steel productis measured in 30 sites, the predicted value of the maximum inclusiondiameter in 30000 mm² as calculated according to statistics of extremevalues is not more than 60 μm, preferably not more than 40 μm, morepreferably not more than 25 μm. The cyclic stress fatigue strength andthe fatigue limit are known to greatly depend upon the maximum inclusiondiameter in a predetermined volume. This is disclosed in Japanese PatentLaid-Open No. 194121/1999 of which the applicant is identical to that inthe application of the present invention. High-cleanliness steels,wherein, for example, typically when the maximum inclusion diameter in100 mm² of the cross-section of the steel product is measured in 30sites, the predicted value of the maximum inclusion diameter in 30000mm² as calculated according to statistics of extreme values is not morethan 60 μm, preferably not more than 40 μm, more preferably not morethan 25 μm, stably exhibit excellent fatigue strength. In this case, thehigh-cleanliness steels have an oxygen content of not more than 10 ppm,preferably not more than 8 ppm in the case of C<0.6% by mass in thesteel, particularly preferably not more than 6 ppm in the case of C≧0.6%by mass, and a predicted value of maximum inclusion diameter of not morethan 60 μm, preferably not more than 40 μm, more preferably not morethan 25 μm. The steels produced by the process according to the presentinvention are high-cleanliness steels possessing both excellent rollingfatigue life and excellent fatigue strength. While acid dissolution is avery time-consuming, troublesome work, the above method, which, withoutsteel product dissolution work, can observe a certain area under amicroscope to statistically predict the maximum inclusion diameter, isadvantageously simple. Further, particularly in fatigue created bycyclic stress of tensile compression, it is known that the maximumdiameter of inclusions present at a site susceptible to failure is agreat factor which governs the strength. This method, which canstatistically predict this maximum diameter, is advantageous.

[0121] Fifth Invention

[0122] A preferred production process of a high-cleanliness steelaccording to the fifth invention comprises the following steps (1) to(5).

[0123] (1) A molten steel is subjected to oxidizing refining in an arcmelting furnace or a converter to prepare a molten steel having apredetermined chemical composition and a predetermined temperature whichis then transferred to a ladle furnace.

[0124] (2) The molten steel transferred to the ladle refining furnace issubjected to reduction refining in the ladle furnace and the chemicalcomposition of the molten steel is regulated. At that time, in the ladlefurnace, an stirring gas is blown through the bottom of the ladle at 1.5to 5.0 N.l/min/t to forcibly agitate the molten steel, and, in addition,electromagnetic stirring is carried out. Thus, ladle refining is carriedout for 50 to 80 min, preferably 70 to 80 min.

[0125] (3) The molten steel, which has been subjected to reductionrefining and regulation of chemical composition in step (2), is degassedby circulating the molten steel through a circulation-type vacuumdegassing device, and, in addition, the chemical composition of thesteel is finally regulated. In this case, it is a general knowledge thatthe degassing time is less than 25 min and, in a circulation-type vacuumdegassing device, satisfactory results are obtained by bringing theamount of the molten steel circulated to about 5 times the total amountof the molten steel. On the other hand, in the present invention, theamount of the molten steel circulated is brought to at least 8 times,preferably at least 10 times, more preferably at least 15 times thetotal amount of the molten steel, and the degassing is carried out for alonger period of time, that is, for not less than 25 min. The steps (2)and (3) are most important to the fifth invention. In the ladle refiningtime for refining while gas stirring and electromagnetic stirring instep (2), even when the refining is not short-time refining, that is,even refining for a long period of time, i.e., 50 to 80 min, preferably70 to 80 min, can also satisfactorily enhance the cleanliness. Thestirring energy of the electromagnetic stirring is brought to 200 to 700w per ton of the molten steel. As described above, the electromagneticstirring does not agitate slag itself. Therefore, it is possible toprevent breaking of the slag equilibrium system caused by melt loss ofrefractories of the furnace and the inclusion of slag. Further, sincedegassing, particularly circulation-type vacuum degassing, is carriedout in such a manner that a nozzle is dipped in the molten steel andonly the molten steel is circulated, the slag on the upper surface ofthe molten steel is in a satisfactorily quiet state, and the number ofoxide inclusions from slag into the molten steel is fewer than thatduring the reduction period process in the ladle. In this system, whenthe floating separation time for oxide inclusions is satisfactorilyensured, an increase in oxygen content caused by contamination fromrefractories or slag on the inner side of the ladle can be preventedand, in addition, the formation of large inclusions having a size of notless than about 30 μm can be prevented. This can realize the productionof high-cleanliness steels.

[0126] (4) The molten steel, which has been subjected to finalregulation of the chemical composition, is cast into an ingot.

[0127] (5) The ingot is press forged into a product shape which is thenoptionally heat treated to provide a steel product.

[0128] In the production process of a high-cleanliness steel, accordingto a preferred embodiment, in the ladle refining in step (2) among thesteps (1) to (5), particularly the ladle is brought to an inertatmosphere and thus is blocked from the air, and, in this state, ladlerefining is carried out (step 6). In this preferred embodiment of thepresent invention, step (6) is most important to the present invention.

[0129] The practice of the ladle refining in an inert atmosphere whileblocking from the air in step (6), in combination of the ladle refiningwherein refining is carried out by gas stirring in combination withelectromagnetic stirring in step (2), permits, even when the refining isnot short-time refining, that is, even refining for a long period oftime, i.e., 50 to 80 min, preferably 70 to 80 min, to satisfactorilyenhance the cleanliness. Specifically, the ladle is covered. The spacedefined by the cover is filled with an inert gas, for example, an argongas, a nitrogen gas, or a mixed gas composed of an argon gas and anitrogen gas to seal the molten steel in the ladle from the air. Thus,the equilibrium system of the slag is maintained. Preferably, thepressure of the inert gas within the cover is reduced to not more than10 Torr. This can further enhance the effect. According to thisconstitution, the slag can be fully floated, and the separation anddropping of the metal and slag into the molten steel in an advancedrefining state during the ladle refining, thereby increasing the oxygencontent, can be prevented. The sealing gas is a gas of not less than 50Nm³/H, and, in the case of refining under reduced pressure, a gas flowrate below this range is also possible.

[0130] The present invention embraces a high-cleanliness steel producedby the above means.

[0131] According to a preferred embodiment, the high-cleanliness steelaccording to the present invention is a high-cleanliness steel,excellent particularly in rolling fatigue life, which is characterizedin that the content of oxygen in the steel is not more than 10 ppm;preferably, when the content of carbon in the steel is less than 0.6% bymass, the content of oxygen in the steel is not more than 8 ppm; and,Particularly preferably, in the case of C≧0.6% by mass, the oxygencontent is not more than 6 ppm. It is generally known that lowering theoxygen content can contribute to improved rolling fatigue life. Amongthe steels produced by the production process according to the presentinvention, high-cleanliness steels having an oxygen content of not morethan 10 ppm, preferably not more than 8 ppm in the case of C<0.6% bymass in the steel, particularly preferably not more than 6 ppm in thecase of C≧0.6% by mass, stably exhibit excellent rolling fatigue life.

[0132] Further, according to a preferred embodiment, the steels producedaccording to the process of the present invention includehigh-cleanliness steels possessing excellent rolling fatigue life andfatigue strength, which are characterized in that the number of oxideinclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid, for example, oxide inclusionshaving an Al₂O₃ content of not less than 50%, is not more than 40,preferably not more than 30, more preferably not more than 20, per 100 gof the steel product. This evaluation method for steel products reflectsboth the oxygen content and the maximum inclusion diameter in apredetermined volume. Regarding the fatigue strength, fatigue life, andquietness, in the case of steels having the same oxygen content, oxideinclusions having a certain large size are harmful, and, in particular,oxide inclusions having a size of not less than 20 μm are harmful.Therefore, among the steels produced by the process according to thepresent invention, steels, wherein the number of oxide inclusions havinga size of not less than 20 μm (for example, having an Al₂O₃ content ofnot less than 50%) as detected by dissolving the steel product in anacid is not more than 40, preferably not more than 30, more preferablynot more than 20, per 100 g of the steel product, are high-cleanlinesssteels having both excellent rolling fatigue life and excellent fatiguestrength and, in addition, excellent quietness.

[0133] According to a preferred embodiment, the steels according to thepresent invention further include high-cleanliness steels, which areexcellent particularly in rotating bending fatigue strength and cyclicstress fatigue strength and are characterized in that, when the maximuminclusion diameter in 100 mm² of the cross-section of the steel productis measured in 30 sites, the predicted value of the maximum inclusiondiameter in 30000 mm² as calculated according to statistics of extremevalues is not more than 60 μm, preferably not more than 40 μm, morepreferably not more than 25 μm. The cyclic stress fatigue strength andthe fatigue limit are known to greatly depend upon the maximum inclusiondiameter in a predetermined volume. This is disclosed in Japanese PatentLaid-Open No. 194121/1999 of which the applicant is identical to that inthe application of the present invention. High-cleanliness steels,wherein, for example, typically when the maximum inclusion diameter in100 mm² of the cross-section of the steel product is measured in 30sites, the predicted value of the maximum inclusion diameter in 30000mm² as calculated according to statistics of extreme values is not morethan 60 μm, preferably not more than 40 μm, more preferably not morethan 25 μm, stably exhibit excellent fatigue strength. In this case, thehigh-cleanliness steels have an oxygen content of not more than 10 ppm,preferably not more than 8 ppm in the case of C<0.6% by mass in thesteel, particularly preferably not more than 6 ppm in the case of C≧0.6%by mass, and a predicted value of maximum inclusion diameter of not morethan 60 μm preferably not more than 40 μm, more preferably not more than25 μm. The steels produced by the process according to the presentinvention are high-cleanliness steels possessing both excellent rollingfatigue life and excellent fatigue strength. While acid dissolution is avery time-consuming, troublesome work, the above method, which, withoutsteel product dissolution work, can observe a certain area under amicroscope to statistically predict the maximum inclusion diameter, isadvantageously simple.

[0134] Further, particularly in fatigue created by cyclic stress oftensile compression, it is known that the maximum diameter of inclusionspresent at a site susceptible to failure is a great factor which governsthe strength. This method, which can statistically predict this maximumdiameter, is advantageous.

EXAMPLE A

[0135] In tapping a molten steel, which had been subjected to oxidizingrefining in an arc melting furnace, from the melting furnace,dexoidizers, such as manganese, aluminum, and silicon, were previouslyadded to a ladle or alternatively were added to the molten steel in thecourse of the tapping. The amount of the deoxidizers added was not lessthan 1 kg on a purity basis per ton of the molten steel to performtapping deoxidation, that is, pre-deoxidation. The molten steel was thensubjected to reduction refining in a ladle refining process, and therefined molten steel was degassed in a circulation-type vacuum degassingdevice, followed by an ingot production process using casting. Steelproducts of JIS SUJ 2 and SCM 435 in 10 heats thus obtained wereexamined for the oxygen content of the products, the predicted value ofthe maximum inclusion diameter according to statistics of extremevalues, and L₁₀ service life by a thrust-type rolling service lift test.In the measurement of the predicted value of the maximum inclusiondiameter, a test piece was taken off from φ65 forged material, theobservation of 100 mm² was carried out for 30 test pieces, and themaximum inclusion diameter in 30000 mm² was predicted according tostatistics of extreme values. In the thrust-type rolling service lifetest, a test piece having a size of φ60×φ20×8.3T, which had beensubjected to carburizing, quench hardening and tempering, was tested ata maximum hertz stress Pmax: 4900 MPa, followed by calculation todetermine the L₁₀ service life.

[0136] An example of operation according to the present invention for 10heats of steel SUJ 2 is shown in Table A1. TABLE A1 Operation Tappingdeoxidation (A₁) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. + ° C.62 56 52 57 65 60 75 65 57 73 Amount of deoxidizer added at the 1.9 32.2 2.8 1.3 1.9 2.9 2 2.8 1 time of tapping or added to ladle, kg/t LF:Time, min 55 51 56 56 60 57 59 57 60 55 LF: Termination temp., ° C. 15251526 1521 1520 1526 1524 1525 1522 1526 1523 RH: Time, min 23 23 23 2323 23 23 23 23 23 RH: Quantity of circulation, times 5.7 6.5 7.1 5.5 6.76.4 5.6 6.8 5.7 7 RH: Termination temp., ° C. 1499 1493 1492 1498 15021502 1492 1497 1500 1499 Casting temp., ° C. 1475 1476 1476 1475 14781478 1475 1477 1476 1475 Oxygen content of product, ppm 4.9 5.6 4.8 5.25.3 5.3 4.9 4.9 5.8 5.1 Number of inclusions of not less 38 33 30 26 2735 32 34 31 36 than 20 μm in 100 g of steel product Maximum predicteddiameter of 49 44.8 38.4 52 47.7 42.4 49 49 52.2 40.8 inclusions, μm L₁₀(× 10⁷) 2.2 1.9 3.1 3.0 2.5 2.4 2.7 3.5 2.9 2.8 Results of evaluation ΔΔ Δ Δ Δ Δ Δ Δ Δ Δ

[0137] An example of the operation according to the present inventionfor 10 heats of steel SCM 435 is shown in Table A2. TABLE A2 OperationTapping deoxidation (B₁) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435Tapping temp.: m.p. + ° C. 68 54 69 61 74 68 62 67 55 65 Amount ofdeoxidizer 2.5 1.8 2.5 1.9 1.5 1.6 1.7 1.5 1.5 2.6 added at the time oftapping or added to ladle, kg/t LF: Time, min 55 51 57 56 59 53 60 53 5451 LF: Termination temp., ° C. 1565 1574 1567 1571 1570 1569 1572 15751565 1573 RH: Time, min 22 22 21 20 23 20 24 23 20 21 RH: Quantity ofcirculation, 6.8 6.0 6.6 5.7 5.9 5.5 7.0 6.5 7.0 6.3 times RH:Termination temp., ° C. 1531 1533 1537 1534 1531 1532 1539 1541 15391536 Casting temp., ° C. 1514 1518 1518 1520 1520 1516 1520 1520 15121516 Oxygen content of product, 7.9 6.7 8.0 7.4 7.9 6.5 8.3 7.9 7.9 6.9ppm Number of inclusions of not 40 33 35 39 35 25 25 30 37 36 less than20 μm in 100 g of steel product Maximum predicted diameter 47.4 46.948.0 51.8 55.3 45.5 49.8 55.3 55.3 45.4 of inclusions, μm L₁₀ (× 10⁷)1.2 1.9 1.8 2.1 1.5 2.8 2.7 1.2 2.4 2.1 Results of evaluation Δ Δ Δ Δ ΔΔ Δ Δ Δ Δ

[0138] An example of the operation according to the present inventionfor 10 heats of steel SUJ 2 is shown in Table A3. TABLE A3 OperationTapping deoxidation + tapping temp. (A₂) No. 1 2 3 4 5 6 7 8 9 10 Typeof steel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2Tapping temp.: m.p. + ° C. 147 148 116 145 155 152 139 113 152 126Amount of deoxidizer added at the 2.7 1.5 2.3 1.7 1.7 2.7 1.9 2.3 1.12.7 time of tapping or added to ladle, kg/t LF: Time, min 56 60 59 51 5353 52 52 58 53 LF: Termination temp., ° C. 1524 1520 1521 1523 1523 15201523 1525 1525 1522 RH: Time, min 23 23 23 23 23 23 23 23 23 23 RH:Quantity of circulation, times 6 6.5 5.5 6.3 5.9 6.7 6.4 6.1 6.7 6.3 RH:Termination temp., ° C. 1498 1501 1502 1500 1503 1498 1502 1497 14941501 Casting temp., ° C. 1478 1476 1476 1476 1477 1476 1478 1475 14781476 Oxygen content of product, ppm 5.2 5.1 5 4.6 4.9 5.1 4.5 5.2 4.94.7 Number of inclusions of not less 30 28 28 26 25 22 23 16 25 30 than20 μm in 100 g of steel product Maximum predicted diameter of 20.8 20.420 23 24.5 25.5 22.5 26 24.5 23.5 inclusions, μm L₁₀ (× 10⁷) 3.4 3.7 4.74.0 4.1 2.6 3.3 4.9 3.9 5.2 Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0139] An example of the operation according to the present inventionfor 10 heats of steel SCM 435 is shown in Table A4. TABLE A4 OperationTapping deoxidation + tapping temp. (B₂) No. 1 2 3 4 5 6 7 8 9 10 Typeof steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 Tapping temp.: m.p. + ° C. 104 119 138 116 119 147 114141 110 113 Amount of deoxidizer 2 2.8 1.9 2.2 2.9 2.5 1.7 1.6 1.5 2.9added at the time of tapping or added to ladle, kg/t LF: Time, min 49 5152 51 52 47 53 51 51 47 LF: Termination temp., ° C. 1565 1572 1572 15721573 1572 1575 1566 1572 1567 RH: Time, min 24 20 22 21 23 20 24 22 2322 RH: Quantity of circulation, 6.5 6.1 5.5 7.2 6.6 6.5 7.1 5.8 7.3 7.0times RH: Termination temp., ° C. 1533 1538 1532 1534 1540 1538 15381536 1538 1538 Casting temp., ° C. 1519 1517 1517 1511 1516 1515 15131516 1511 1513 Oxygen content of product, 7.1 7.3 7.1 7.4 6.5 6.8 7.17.1 6.9 6.4 ppm Number of inclusions of not 28 29 20 25 30 28 29 26 2220 less than 20 μm in 100 g of steel product Maximum predicted diameter37.6 38.5 38.3 39.3 34.5 35.6 37.8 36.2 34.5 32.6 of inclusions, μm L₁₀(× 10⁷) 2.9 2.8 2.4 3.0 3.6 3.3 3.4 3.1 2.8 3.3 Results of evaluation ◯◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0140] An example of the operation of tapping deoxidation+short LF, longRH according to the present invention for 10 heats of steel SUJ 2 isshown in Table A5. TABLE A5 Operation Tapping deoxidation + short LF,long RH (A₃) No. 1 2 3 4 5 6 7 8 9 10 Type of Steel SUJ 2 SUJ 2 SUJ 2SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. + ° C. 6680 61 79 55 66 68 65 67 60 Amount of deoxidizer added at the 1.8 1.7 31.6 2.6 2.7 2.8 2.2 3 2 time of tapping or added to ladle, kg/t LF:Time, min 41 34 33 31 38 30 40 32 39 44 LF: Termination temp., ° C. 15461547 1548 1549 1550 1551 1552 1553 1554 1555 RH: Time, min 56 57 59 5455 55 54 57 60 58 RH: Quantity of circulation, times 18.7 19.0 19.7 18.018.3 18.3 18.0 19.0 20.0 19.3 RH: Termination temp., ° C. 1502 1510 15061502 1505 1508 1503 1508 1506 1508 Casting temp., ° C. 1478 1477 14771478 1477 1478 1478 1475 1477 1476 Oxygen content of product, ppm 4.8 44.1 4.6 5.2 4.8 4.5 4.2 4.2 4.4 Number of inclusions of not less 26 3022 28 21 20 30 30 26 23 than 20 μm in 100 g of steel product Maximumpredicted diameter of 21.8 19.4 18.9 21 21.6 18.4 22.7 21.3 20.8 20.2inclusions, μm L₁₀ (× 10⁷) 4.8 4.0 5.1 4.0 3.4 3.9 4.4 3.6 3.7 3.1Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0141] An example of the operation of tapping deoxidation+short LF, longRH according to the present invention for 10 heats of steel SCM 435 isshown in Table A6. TABLE A6 Operation Tapping deoxidation + short LF,long RH (B₃) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Tappingtemp.: m.p. + ° C. 62 72 56 55 71 59 63 78 67 63 Amount of deoxidizer 31.6 2.8 1.8 2.9 2.4 2.3 2.6 2.1 1.9 added at the time of tapping oradded to ladle, kg/t LF: Time, min 42 42 40 41 42 45 41 37 42 36 LF:Termination temp., ° C. 1580 1582 1585 1580 1579 1578 1578 1585 15841581 RH: Time, min 36 45 39 35 43 39 45 36 43 38 RH: Quantity of 12.015.0 13.0 11.7 14.3 13.0 15.0 12.0 14.3 12.7 circulation, times RH:Termination temp., ° C. 1537 1533 1533 1535 1539 1539 1534 1539 15341539 Casting temp., ° C. 1514 1513 1515 1515 1515 1516 1516 1515 15161515 Oxygen content of product, 7 7.3 7.2 7.1 6.7 7.3 6.8 7.1 6.5 7.1ppm Number of inclusions of 28 29 25 25 22 30 23 28 26 23 not less than20 μm in 100 g of steel product Maximum predicted diameter 25.0 25.024.9 24.7 25.0 24.8 24.9 24.6 24.7 24.9 of inclusions, μm L₁₀ (× 10⁷)3.0 2.6 3.8 3.7 3.1 3.3 2.9 2.3 3.6 2.7 Results of evaluation ◯ ◯ ◯ ◯ ◯◯ ◯ ◯ ◯ ◯

[0142] An example of the operation of tappingdeoxidation+high-temperature tapping+short LF, long RH according to thepresent invention for 10 heats of steel SUJ 2 is shown in Table A7.TABLE A7 Operation Tapping deoxidation + tapping temp. + shrot LF, longRH (A₄) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ 2 SUJ 2SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. + ° C. 132 143131 150 153 134 151 138 111 157 Amount of deoxidizer added at the 2.8 12.9 1.9 2.7 2.6 2.5 2.4 1.7 2.2 time of tapping or added to ladle, kg/tLF: Time, min 43 34 35 38 31 39 38 41 35 44 LF: Termination temp., ° C.1541 1541 1546 1546 1541 1540 1543 1544 1544 1546 RH: Time, min 54 50 5848 52 47 51 60 53 48 RH: Quantity of circulation, times 18.8 16.1 18.616.0 16.8 15.7 17.6 20.7 18.2 16.5 RH: Termination temp., ° C. 1498 15021502 1502 1500 1501 1498 1502 1497 1498 Casting temp., ° C. 1478 14761477 1475 1478 1475 1475 1476 1476 1475 Oxygen content of product, ppm4.1 4.7 4.1 4.2 4.1 4.9 4.3 3.8 4.3 4.7 Number of inclusions of not less14 11 5 6 8 8 13 10 6 7 than 20 μm in 100 g of steel product Maximumpredicted diameter of 12.3 14.1 12.3 14.4 14.1 14.7 12.9 11.4 12.9 13.8inclusions, μm L₁₀ (× 10⁷) 7.1 7.9 9.9 9.1 11.3 10.6 10.9 11.9 10.0 8.4Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0143] An example of the operation of tappingdeoxidation+high-temperature tapping+short LF, long RH according to thepresent invention for 10 heats of steel SCM 435 is shown in Table A8.TABLE A8 Operation Tapping deoxidation + tapping temp. + short LF, longRH (B₄) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Tapping temp.:m.p. + ° C. 143 115 104 148 130 106 109 124 122 105 Amount of deoxidizeradded 2 2.1 2.4 1.7 1.7 2.9 2.1 2 2.4 2.5 at the time of tapping oradded to ladle, kg/t LF: Time, min 35 34 33 42 33 43 38 45 41 37 LF:Termination temp., ° C. 1577 1579 1585 1578 1584 1578 1582 1581 15771576 RH: Time, min 36 45 44 40 38 37 46 39 40 43 RH: Quantity ofcirculation, 12.4 14.5 14.2 13.3 13.1 11.9 15.3 13.0 12.9 14.3 times RH:Termination temp., ° C. 1532 1541 1535 1537 1531 1531 1532 1540 15381536 Casting temp., ° C. 1513 1520 1517 1521 1516 1511 1518 1511 15111519 Oxygen content of product, 6.5 5.4 5.5 5.9 6.0 6.1 5.3 6.0 5.8 5.7ppm Number of inclusions of not 8 10 6 9 8 14 8 14 11 8 less than 20 μmin 100 g of steel product Maximum predicted diameter 24.6 23.5 23.8 24.424.6 24.0 22.5 24.0 26.7 26.8 of inclusions, μm L₁₀ (× 10⁷) 7.9 8.6 10.49.3 9.8 9.6 8.8 8.7 10.0 9.3 Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0144] For comparison with the present invention, an example of theoperation according to a prior art technique for steel SUJ 2 is shown inTable A9, and an example of the operation according to a prior arttechnique for steel SCM 435 is shown in Table A10. TABLE A9 OperationConventional operation (prior art) No. 1 2 3 4 5 6 7 8 9 10 Type ofsteel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2Tapping temp.: m.p. + ° C. 57 72 58 60 74 75 51 65 62 68 Amount ofdeoxidizer added at the — — — — — — — — — — time of tapping or added toladle, kg/t LF: Time, min 61 61 63 61 62 62 61 63 61 63 LF: Terminationtemp., ° C. 1525 1524 1526 1525 1523 1524 1523 1520 1525 1520 RH: Time,min 23 23 23 23 23 23 23 23 23 23 RH: Quantity of circulation, times 5.76.7 7.1 6.5 6.2 5.7 7 5.5 6.8 6.2 RH: Termination temp., ° C. 1493 15021501 1497 1501 1501 1502 1503 1496 1499 Casting temp., ° C. 1477 14751475 1475 1475 1475 1476 1478 1478 1476 Oxygen content of product, ppm5.4 5.1 5.1 6.1 5.8 5.9 5.8 5.9 5.2 6.2 Number of inclusions of not less59 56 54 65 48 41 50 47 45 49 than 20 μm in 100 g of steel productMaximum predicted diameter of 86.4 61.2 66.3 97.6 81.2 76.7 92.8 76.772.8 74.4 inclusions, μm L₁₀ (× 10⁷) 1.9 2.4 2.4 1.8 1.9 3.4 1.9 2.2 2.02.2 Results of evaluation x x x x x x x x x x

[0145] TABLE A10 Operation Conventional operation (prior art) No. 1 2 34 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM435 SCM 435 SCM 435 SCM 435 SCM 435 Tapping temp.: m.p. + ° C. 61 54 6950 74 58 58 69 64 54 Amount of deoxidizer — — — — — — — — — — added atthe time of tapping or added to ladle, kg/t LF: Time, min 62 63 61 61 6163 63 63 61 61 LF: Termination temp., ° C. 1570 1574 1566 1572 1567 15691567 1569 1569 1570 RH: Time, min 23 23 23 20 21 23 21 23 23 24 RH:Quantity of circulation, 6.8 7.5 7.0 8.3 6.2 6.0 7.4 8.0 7.3 6.7 timesRH: Termination temp., ° C. 1533 1538 1541 1540 1541 1533 1535 1534 15311531 Casting temp., ° C. 1517 1519 1520 1518 1517 1511 1516 1512 15121521 Oxygen content of product, 7.6 9.2 9.2 8.8 6.9 8.3 6.9 8.3 9.4 9.1ppm Number of inclusions of not 49 54 59 52 42 57 56 53 53 42 less than20 μm in 100 g of steel product Maximum predicted diameter 68.4 82.873.6 70.4 55.2 83.0 55.2 83.0 84.6 91.0 of inclusions, μm L₁₀ (× 10⁷)1.0 1.3 1.1 1.9 2.3 1.5 2.0 1.2 1.2 1.9 Results of evaluation x x x x xx x x x x

[0146] As is apparent from Tables A1 to A8, for steel products producedusing tapping deoxidation, that is, pre-deoxidation, according to thepresent invention, when the tapping temperature is brought to a hightemperature above the conventional operation, that is, the meltingpoint+at least 100° C., and, in addition, degassing is satisfactorilycarried out by shortening the operation time in the ladle refiningfurnace and, in addition, increasing the quantity of circulation RH incirculation degassing (that is, amount of molten steel circulated/totalamount of molten steel), for both steel types, SUJ 2 and SCM 435, theoxygen content of the products is small and, in addition, the number ofinclusions having a size of not less than 20 μm is significantlydecreased. As can be seen from Tables A1 to A8, regarding thecleanliness, for the examples of the present invention, all the steelproducts are evaluated as fair (Δ), good (◯), and excellent (⊚), thatis, are excellent high-cleanliness steels. By contrast, as can be seenfrom Tables A9 and A10, for all the conventional examples, thecleanliness is evaluated as failure (X), and the conventional steelproducts cannot be said to be clean steels. In this connection, itshould be noted that fair (Δ) is based on the comparison with good (◯)and excellent (⊚) and, as compared with steels not subjected to tappingdeoxidation according to the prior art method which is evaluated asfailure (X), the steels evaluated as fair (Δ) have much highercleanliness.

[0147] For heats wherein pre-deoxidation, that is, tapping deoxidation,has been carried out, both the oxygen content and the predicted value ofthe maximum inclusion diameter are reduced by increasing T_(SH)[(temperature at which molten steel is transferred to ladlefurnace)−(melting point of molten steel)=T_(SH))] to improve thecleanliness. For heats in which pre-deoxidation has been carried out,regarding the relationship of the refining time in the ladle furnacewith the oxygen content and the predicted value of the maximum inclusiondiameter, when the refining time is not less than about 25 min, theoxygen content and the predicted value of the maximum inclusion diameterare satisfactorily lowered. The predicted value of the maximum inclusiondiameter, however, increases with increasing the refining time. Thereason for this is considered as follows. With the elapse of time, themelt loss of refractories in the ladle furnace is increased, theequilibrium of the slag system is broken, for example, as a result ofoxidation due to the contact with the air, and the level of thedissolved oxygen goes beyond the minimum level of dissolved oxygen.Further, the relationship of the amount of molten steel circulated/totalamount of molten steel in the circulation-type vacuum degassing devicewith the oxygen content and the predicted value of the maximum inclusiondiameter, the effect of enhancing the cleanliness increases withincreasing the amount of molten steel circulated, and is substantiallysaturated when the amount of molten steel circulated/total amount ofmolten steel is not less than 15 times.

[0148] It was confirmed that reducing the oxygen content and thepredicted value of the maximum inclusion diameter results in improvedL₁₀ life. This indicates that steels produced by the process accordingto the present invention, which can reduce the oxygen content and thepredicted value of the maximum inclusion diameter, have excellentfatigue strength properties such as excellent rolling fatigue life.

[0149] FIG. A1 is a diagram showing the oxygen content of products in 10heats in the production process according to the present inventionwherein the tapping deoxidation is performed in the transfer of themolten steel of steel SUJ 2 to the ladle furnace, and the oxygen contentof products in 10 heats in the conventional process wherein the tappingdeoxidation is not carried out. In FIGS. A1, A3, and A5, A₁ shows dataon the tapping deoxidation according to the present invention defined inclaim 1, A₂ data on the, tapping deoxidation+high-temperature tappingaccording to the present invention defined in claim 2, A₃ data on thetapping deoxidation+short-time LF, long-time RH treatment according tothe present invention defined in claim 3, A₄ data on the tappingdeoxidation+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention defined in claim 3, andconventional data on prior art.

[0150] FIG. A2 is a diagram showing the oxygen content of products in 10heats in the production process according to the present inventionwherein the tapping deoxidation is performed in the transfer of themolten steel of steel SCM 435 to the ladle, and the oxygen content ofproducts in 10 heats in the conventional process wherein the tappingdeoxidation is not carried out. In FIGS. A2, A4, and A6, B₁ shows dataon the tapping deoxidation according to the present invention defined inclaim 1, B₂ data on the tapping deoxidation+high-temperature tappingaccording to the present invention defined in claim 2, B3 data on thetapping deoxidation+short-time LF, long-time RH treatment according tothe present invention defined in claim 3, B4 data on the tappingdeoxidation+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention defined in claim 3, andconventional data on prior art.

[0151] FIG. A3 is a diagram showing the maximum predicted inclusiondiameter determined according to statistics of extreme values in 10heats in the production process according to the present inventionwherein the deoxidation is performed in the transfer of the molten steelof steel SUJ 2 to the ladle furnace, and according to the prior artmethod wherein the deoxidation is not carried out.

[0152] FIG. A4 is a diagram showing the maximum predicted inclusiondiameter determined according to statistics of extreme values in 10heats in the production process according to the present inventionwherein the deoxidation is performed in the transfer of the molten steelof steel SCM 435 to the ladle furnace, and according to the prior artmethod wherein the deoxidation is not carried out.

[0153] FIG. A5 shows data on L₁₀ life as determined by a thrust rollingservice life test in 10 heats in the production process according to thepresent invention wherein the deoxidation is performed in the transferof the molten steel of steel SUJ 2 to the ladle furnace, and accordingto the prior art method wherein the deoxidation is not carried out.

[0154] FIG. A6 shows data on L₁₀ life as determined by a thrust rollingservice life test in 10 heats in the production process according to thepresent invention wherein the deoxidation is performed in the transferof the molten steel of steel SCM 435 to the ladle furnace, and accordingto the prior art method wherein the deoxidation is not carried out.

[0155] As is apparent from the test results, it was confirmed that, forboth steel SUJ 2 and steel SCM 435, pre-deoxidation, that is, tappingdeoxidation before the ladle refining, can significantly reduce theoxygen content of the products, and the predicted value of the maximuminclusion diameter and, according to the process according to thepresent invention, the cleanliness is significantly improved and the L₁₀life as determined by the thrust rolling service life test issignificantly improved. The addition of treatments to the process, thatis, the addition of only tapping deoxidation according to the presentinvention as defined in claim 1, the addition of tappingdeoxidation+high-temperature tapping according to the present inventiondefined in claim 2, the addition of tapping deoxidation+short-time LF,long-time RH treatment according to the present invention defined inclaim 3, and the addition of the tapping deoxidation+high-temperaturetapping+short-time LE, long-time RH treatment, can significantly improveall the oxygen content of products, the predicted value of the maximuminclusion diameter, and the L₁₀ life as determined by the thrust rollingservice life test. In particular, the addition of short-time LF,long-time RH treatment can offer very large effect.

[0156] As is apparent from the foregoing description, tappingdeoxidation, wherein deoxidizers, such as manganese, aluminum, andsilicon, are previously added to a ladle in the transfer of a moltensteel, produced in a refining furnace, such as an arc furnace, to theladle, or alternatively, is added to the molten steel in the course ofthe transfer of the molten steel to the ladle according to theproduction process of the present invention, whereby the molten steel ispre-deoxidized before the ladle refining, a large quantity of steelproducts having a very high level of cleanliness can be provided withoutuse of a remelting process which incurs very high cost. Further, theadoption of tapping deoxidation+high-temperature tapping and theaddition of tapping deoxidation+high-temperature tapping+short-time LF,long-time RH can provide steel products having a higher level ofcleanliness. This can realize the provision of high-cleanliness steelsfor use as steels for mechanical parts required to possess fatiguestrength, fatigue life, and quietness, particularly, for example, assteels for rolling bearings, steels for constant velocity joints, steelsfor gears, steels for continuously variable transmission of toroidaltype, steels for mechanical structures for cold forging, tool steels,and spring steels, and processes for producing the same, that is, canoffer unprecedented excellent effect.

EXAMPLE B

[0157] A molten steel, which had been produced by a melting process inan arc melting furnace, was circulated through a circulation-type vacuumdegassing device to degas the molten steel. The degassed molten steelwas then transferred to a ladle furnace where the molten steel wassubjected to ladle refining. The refined molten steel was thencirculated through a circulation-type vacuum degassing device to degasthe molten steel, followed by an ingot production process using casting.Steel products of JIS SUJ 2 and SCM 435 in 10 heats thus obtained wereexamined for the oxygen content of the products, the predicted value ofthe maximum inclusion diameter according to statistics of extremevalues, and L₁₀ service life by a thrust-type rolling service lift test.In the measurement of the predicted value of the maximum inclusiondiameter, a test piece was taken off from a φ65 forged material, theobservation of 100 mm² was carried out for 30 test pieces, and themaximum inclusion diameter in 30000 mm² was predicted according tostatistics of extreme values. In the thrust-type rolling service lifetest, a test piece having a size of φ60×φ20×8.3T, which had beensubjected to carburizing, quench hardening and tempering, was tested ata maximum hertz stress Pmax: 4900 MPa, followed by calculation todetermine the L₁₀ service life.

[0158] An example of operation in the case of only W-RH treatmentdefined in claim 1 according to the present invention for 10 heats ofsteel SUJ 2 is shown in Table B1. TABLE B1 Operation W - RH (A₁) No. 1 23 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. + ° C. 75 64 63 60 71 61 73 59 6468 1st RH: Time, min 15 9 15 8 10 8 11 12 15 11 1st RH: Quantity ofcirculation, 5.0 3.0 5.0 2.7 3.3 2.7 3.7 4.0 5.0 3.7 times 1st RH:Amount of deoxidizer added, 2.6 1.6 2.6 1.7 2.8 2 2.9 1.1 1.3 2.6 kg/tLF: Time, min 48 60 49 52 59 57 58 49 48 57 LF: Termination temp., ° C.1532 1534 1533 1532 1528 1531 1533 1534 1535 1533 2nd RH: Time, min 2221 22 25 24 24 25 23 24 25 2nd RH: Quantity of circulation, 7.3 7.0 7.38.3 8.0 8.0 8.3 7.7 8.0 8.3 times 2nd RH: Termination temp., ° C. 15091508 1503 1510 1510 1509 1504 1505 1503 1506 Casting temp., ° C. 14761478 1476 1476 1478 1476 1477 1476 1475 1476 Oxygen content of product,ppm 4.8 5.1 4.6 4.7 4.9 5.1 4.9 4.8 4.8 5 Number of inclusions of notless 23 21 19 26 27 30 21 20 20 29 than 20 μm in 100 g of steel productMaximum predicted diameter of 22.8 20.5 19.7 21.8 20 19.8 19.8 21.2 18.620.2 inclusions, μm L₁₀ (× 10⁷) 3.8 3.3 5.0 4.8 4.7 4.1 5.3 3.2 5.5 4.9Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0159] An example of operation in the case of only W-RH treatmentaccording to the present invention for 10 heats of steel SCM 435 isshown in Table B2. TABLE B2 Operation W - RH (B₁) No. 1 2 3 4 5 6 7 8 910 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 SCM 435 Tapping temp.: m.p. + ° C. 68 74 69 74 65 77 6360 58 70 1st RH: Time, min 12 12 11 12 10 10 13 8 15 15 1st RH: Quantityof 4.0 4.0 3.7 4.0 3.3 3.3 4.3 2.7 5.0 5.0 circulation, times 1st RH:Amount of 2.9 2.2 2 1.5 1.5 1.8 2.3 2.5 2.7 2.2 deoxidizer added, kg/tLF: Time, min 60 47 55 47 56 57 51 45 60 56 LF: Termination temp., ° C.1579 1585 1578 1583 1580 1578 1580 1579 1582 1583 2nd RH: Time, min 2222 25 24 22 25 20 22 25 24 2nd RH: Quantity of 7.3 7.3 8.3 8.0 7.3 8.36.7 7.3 8.3 8.0 circulation, times 2nd RH: Termination temp., 1523 15221523 1524 1525 1521 1524 1520 1524 1522 ° C. Casting temp., ° C. 15151516 1515 1513 1514 1515 1515 1514 1516 1515 Oxygen content of product,6.7 6.7 7 7.2 7.1 6.9 6.6 6.8 6.4 7 ppm Number of inclusions of not 3027 25 22 24 28 23 26 26 26 less than 20 μm in 100 g of steel productMaximum predicted diameter 20.1 21.7 22.8 20.2 24 21.9 22.2 22.5 20.7 22of inclusions, μm L₁₀ (× 10⁷) 2.7 3.3 3.4 2.6 2.5 3.4 4.0 4.0 3.8 3.7Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0160] An example of the operation of W-RH treatment+high-temperaturetapping according to the present invention for 10 heats of steel SUJ 2is shown in Table B3. TABLE B3 Operation W - RH + tapping temp. (A₂) No.1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2SUJ 2 SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. + ° C. 136 152 128 169 163145 120 125 160 154 1st RH: Time, min 15 9 15 8 10 8 11 12 15 11 1st RH:Quantity of circulation, 5.0 3.0 5.0 2.7 3.3 2.7 3.7 4.0 5.0 3.7 times1st RH: Amount of deoxidizer added, 2.6 1.6 2.6 1.7 2.8 2 2.9 1.1 1.32.6 kg/t LF: Time, min 72 64 63 72 72 62 66 60 65 71 LF: Terminationtemp., ° C. 1532 1534 1533 1532 1528 1531 1533 1534 1535 1533 2nd RH:Time, min 22 21 22 24 24 24 23 23 24 24 2nd RH: Quantity of circulation,7.3 7.0 7.3 8.3 8.0 8.0 8.3 7.7 8.0 8.3 times 2nd RH: Termination temp.,° C. 1509 1508 1503 1510 1510 1509 1504 1505 1503 1506 Casting temp., °C. 1476 1478 1476 1476 1478 1476 1477 1476 1475 1476 Oxygen content ofproduct, ppm 4.8 5.1 4.5 4.6 4.9 5.2 5.0 4.6 4.8 5.1 Number ofinclusions of not less 21 23 14 16 20 23 22 17 19 26 than 20 μm in 100 gof steel product Maximum predicted diameter of 15.7 16.2 14.1 14.3 15.616.6 16.0 14.9 14.8 17.2 inclusions, μm L₁₀ (× 10⁷) 7.0 6.0 8.8 7.7 6.55.2 6.6 8.4 7.2 5.3 Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0161] An example of the operation W-RH treatment+high-temperaturetapping according to the present invention for 10 heats of steel SCM 435is shown in Table B4. TABLE B4 Operation W - RH + tapping temp. (B₂) No.1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Tapping temp.: m.p. + ° C.135 140 130 123 102 122 118 109 157 115 1st RH: Time, min 12 12 11 12 1010 13 8 15 15 1st RH: Quantity of 4.0 4.0 3.7 4.0 3.3 3.3 4.3 2.7 5.05.0 circulation, times 1st RH: Amount of 2.9 2.2 2 1.5 1.5 1.8 2.3 2.52.7 2.2 deoxidizer added, kg/t LF: Time, min 72 68 62 71 61 67 64 73 6268 LF: Termination temp., ° C. 1579 1585 1578 1583 1580 1578 1580 15791582 1583 2nd RH: Time, min 22 22 23 24 22 23 20 22 24 24 2nd RH:Quantity of 7.3 7.3 8.3 8.0 7.3 8.3 6.7 7.3 8.3 8.0 circulation, times2nd RH: Termination temp., 1523 1522 1523 1524 1525 1521 1524 1520 15241522 ° C. Casting temp., ° C. 1515 1516 1515 1513 1514 1515 1515 15141516 1515 Oxygen content of product, 6.2 6.7 6.6 6.1 6.3 6.4 6.2 6.5 6.46.5 ppm Number of inclusions of not 14 18 15 13 16 16 13 17 15 18 lessthan 20 μm in 100 g of steel product Maximum predicted diameter 20.221.6 20.3 19.7 20.4 20.8 19.5 21.3 20.6 21.0 of inclusions, μm L₁₀ (×10⁷) 6.2 5.0 6.4 7.8 5.2 6.9 7.0 4.8 5.9 4.1 Results of evaluation ◯ ◯ ◯◯ ◯ ◯ ◯ ◯ ◯ ◯

[0162] An example of the operation of W-RH treatment+short LF, long RHaccording to the present invention for 10 heats of steel SUJ 2 is shownin Table B5. TABLE B5 Operation W - RH + short LF, long RH (A₃) No. 1 23 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. + ° C. 59 68 74 61 69 78 74 59 7367 1st RH: Time, min 14 12 12 9 10 9 12 9 15 11 1st RH: Quantity ofcirculation, 4.7 4.0 4.0 3.0 3.3 3.0 4.0 3.0 5.0 3.7 times 1st RH:Amount of deoxidizer added, 2.6 1.3 1.5 2.2 1 2.2 1.5 2.1 2.2 1.3 kg/tLF: Time, min 44 38 35 44 45 42 41 36 36 44 LF: Termination temp., ° C.1541 1545 1544 1543 1542 1541 1541 1543 1541 1544 2nd RH: Time, min 4938 37 46 54 54 53 59 45 41 2nd RH: Quantity of circulation, 16.3 12.712.3 15.3 18.0 18.0 17.7 19.7 15.0 13.7 times 2nd RH: Termination temp.,° C. 1507 1505 1507 1507 1506 1503 1504 1505 1508 1508 Casting temp., °C. 1476 1478 1478 1476 1475 1475 1477 1477 1476 1476 Oxygen content ofproduct, ppm 4.8 4.3 4.4 4.5 5.1 5.1 4.1 4.4 4.9 4.6 Number ofinclusions of not less 15 14 21 17 25 19 16 12 20 19 than 20 μm in 100 gof steel product Maximum predicted diameter of 14.1 13.7 14.1 13.2 12.514.3 13.8 12.5 12.8 14.7 inclusions, μm L₁₀ (× 10⁷) 8.6 10.6 10.7 10.07.0 9.3 9.9 9.4 8.9 9.4 Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0163] An example of the operation of W-RH treatment+short LF, long RHaccording to the present invention for 10 heats of steel SCM 435 isshown in Table B6. TABLE B6 Operation W - RH + short LF, long RH (B₃)No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Tapping temp.: m.p. + °C. 56 70 78 67 76 63 74 63 64 72 1st RH: Time, min 9 14 12 12 15 13 8 1415 10 1st RH: Quantity of 3.0 4.7 4.0 4.0 5.0 4.3 2.7 4.7 5.0 3.3circulation, times 1st RH: Amount of deoxidizer 2.4 2.8 1.6 2.7 2.2 32.5 3 2.9 1.9 added, kg/t LF: Time, min 40 38 42 41 37 42 36 43 38 35LF: Termination temp., ° C. 1585 1578 1581 1579 1582 1579 1585 1583 15771577 2nd RH: Time, min 31 55 34 32 31 54 37 53 52 46 2nd RH: Quantity of10.3 18.3 11.3 10.7 10.3 18.0 12.3 17.7 17.3 15.3 circulation, times 2ndRH: Termination temp., 1524 1520 1523 1524 1524 1522 1525 1525 1524 1523° C. Casting temp., ° C. 1516 1513 1514 1515 1515 1515 1515 1516 15161514 Oxygen content of product, 6.3 6.4 6.1 6.4 6 6.5 6.5 6.4 6.4 6.4ppm Number of inclusions of not 14 12 11 15 14 15 10 14 11 15 less than20 μm in 100 g of steel product Maximum predicted diameter 24 22.7 22.222.2 23 23.7 23.7 22.5 23.4 22.1 of inclusions, μm L₁₀ (× 10⁷) 7.9 8.810.1 9.7 7.7 6.9 8.3 9.4 9.5 8.0 Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚⊚

[0164] An example of the operation of W-RH treatment+high-temperaturetapping+short LF, long RH according to the present invention for 10heats of steel SUJ 2 is shown in Table B7. TABLE B7 Operation W - RH +tapping temp. + short LF, long RH (A₄) No. 1 2 3 4 5 6 7 8 9 10 Type ofsteel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2Tapping temp.: m.p. + ° C. 140 182 170 149 189 166 163 182 142 157 1stRH: Time, min 13 14 8 13 8 17 15 18 14 11 1st RH: Quantity ofcirculation, 4.3 4.7 2.7 4.3 2.7 5.7 5.0 6.0 4.7 3.7 times 1st RH:Amount of deoxidizer added, 1.2 2.2 0.5 2.1 2.1 1.6 2.5 2.4 0.9 1.1 kg/tLF: Time, min 37 40 40 43 37 37 44 38 33 39 LF: Termination temp., ° C.1541 1546 1546 1543 1540 1545 1542 1544 1540 1542 2nd RH: Time, min 4956 53 59 53 55 46 49 58 56 2nd RH: Quantity of circulation, 15.8 19.217.1 19.7 17.6 18.3 15.7 15.9 20.0 19.4 times 2nd RH: Termination temp.,° C. 1501 1502 1496 1493 1502 1499 1492 1495 1501 1501 Casting temp., °C. 1477 1478 1475 1477 1478 1477 1478 1475 1476 1476 Oxygen content ofproduct, ppm 4.6 4.1 4.5 4 4.3 4.2 3.7 4.5 3.8 3.9 Number of inclusionsof not less 2 5 6 7 8 8 8 5 2 4 than 20 μm in 100 g of steel productMaximum predicted diameter of 11.7 11 11.8 10.9 10.5 10.3 11.2 12.1 10.910.4 inclusions, μm L₁₀ (× 10⁷) 9.7 12.2 11.0 12.6 11.3 10.9 11.5 10.210.8 11.1 Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0165] An example of the operation of W-RH treatment+high-temperaturetapping+short LF, long RH according to the present invention for 10heats of steel SCM 435 is shown in Table B8. TABLE B8 Operation W - RH +tapping temp. + short LF, long RH (B₄) No. 1 2 3 4 5 6 7 8 9 10 Type ofsteel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 Tapping temp.: m.p. + ° C. 136 131 137 106 107 102 136138 105 134 1st RH: Time, min 18 8 9 16 11 8 17 8 15 14 1st RH: Quantityof 6 2.67 3.00 5.33 3.67 2.67 5.67 2.67 5.00 4.67 circulation, times 1stRH: Amount of deoxidizer 2.4 2.1 1 2.5 1.3 1.6 0.8 1.4 0.8 2.3 added,kg/t LF: Time, min 33 37 44 42 40 35 39 40 34 34 LF: Termination temp.,° C. 1577 1581 1577 1576 1579 1586 1582 1585 1579 1584 2nd RH: Time, min39 39 42 42 40 44 37 39 38 41 2nd RH: Quantity of 13.0 13.5 14.0 13.512.4 14.3 12.7 13.3 12.2 12.9 circulation, times 2nd RH: Terminationtemp., 1541 1538 1532 1539 1541 1537 1540 1537 1532 1539 ° C. Castingtemp., ° C. 1515 1518 1521 1513 1518 1520 1521 1519 1511 1520 Oxygencontent of product, 6.0 5.8 5.3 5.2 5.6 4.7 5.5 5.5 5.8 5.6 ppm Numberof inclusions of not 5 3 6 8 8 6 2 5 4 3 less than 20 μm in 100 g ofsteel product Maximum predicted diameter 22.0 21.3 20.3 20.5 23.4 20.022.9 22.1 23.2 21.8 of inclusions, μm L₁₀ (× 10⁷) 10.4 10.6 9.8 9.6 10.011.0 9.2 9.1 10.2 9.9 Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0166] For comparison with the present invention, an example of theoperation according to a prior art technique for steel SUJ 2 is shown inTable B9, and an example of the operation according to a prior arttechnique for steel SCM 435 is shown in Table B10. TABLE B9 OperationConventional operation (prior art) No. 1 2 3 4 5 6 7 8 9 10 Type ofsteel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2Tapping temp.: m.p. + ° C. 57 72 58 60 74 75 51 65 62 68 1st RH: Time,min — — — — — — — — — — 1st RH: Quantity of circulation, — — — — — — — —— — times 1st RH: Amount of deoxidizer added, — — — — — — — — — — kg/tLF: Time, min 61 61 63 61 62 62 61 63 61 63 LF: Termination temp., ° C.1525 1524 1526 1525 1523 1524 1523 1520 1525 1520 2nd RH: Time, min 2323 23 23 23 23 23 23 23 23 2nd RH: Quantity of circulation, 5.7 6.7 7.16.5 6.2 5.7 7 5.5 6.8 6.2 times 2nd RH: Termination temp., ° C. 14931502 1501 1497 1501 1501 1502 1503 1496 1499 Casting temp., ° C. 14771475 1475 1475 1475 1475 1476 1478 1478 1476 Oxygen content of product,ppm 5.4 5.1 5.1 6.1 5.8 5.9 5.8 5.9 5.2 6.2 Number of inclusions of notless 59 56 54 65 48 41 50 47 45 49 than 20 μm in 100 g of steel productMaximum predicted diameter of 86.4 61.2 66.3 97.6 81.2 76.7 92.8 76.772.8 74.4 inclusions, μm L₁₀ (× 10⁷) 1.9 2.4 2.4 1.8 1.9 3.4 1.9 2.2 2.02.2 Results of evaluation x x x x x x x x x x

[0167] TABLE B10 Operation Conventional operation (prior art) No. 1 2 34 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM435 SCM 435 SCM 435 SCM 435 SCM 435 Tapping temp.: m.p. + ° C. 61 54 6950 74 58 58 69 64 54 1st RH: Time, min — — — — — — — — — — 1st RH:Quantity of — — — — — — — — — — circulation, times 1st RH: Amount ofdeoxidizer — — — — — — — — — — added, kg/t LF: Time, min 62 63 61 61 6163 63 63 61 61 LF: Termination temp., ° C. 1570 1574 1566 1572 1567 15691567 1569 1569 1570 2nd RH: Time, min 23 23 23 20 21 23 21 23 23 24 2ndRH: Quantity of 6.8 7.5 7.0 8.3 6.2 6.0 7.4 8.0 7.3 6.7 circulation,times 2nd RH: Termination temp., 1533 1538 1541 1540 1541 1533 1535 15341531 1531 ° C. Casting temp., ° C. 1517 1519 1520 1518 1517 1511 15161512 1512 1521 Oxygen content of product, 7.6 9.2 9.2 8.8 6.9 8.3 6.98.3 9.4 9.1 ppm Number of inclusions of not 49 54 59 52 42 57 56 53 5342 less than 20 μm in 100 g of steel product Maximum predicted diameter68.4 82.8 73.6 70.4 55.2 83.0 55.2 83.0 84.6 91.0 of inclusions, μm L₁₀(× 10⁷) 1.0 1.3 1.1 1.9 2.3 1.5 2.0 1.2 1.2 1.9 Results of evaluation xx x x x x x x x x

[0168] As is apparent from Tables B1 to B8, for steel products producedusing W-RH treatment according to the present invention wherein a moltensteel produced in an arc melting furnace or a converter is pre-degassed,is transferred to a ladle furnace to perform refining, and is thencirculated through a circulation-type vacuum degassing device to degasthe molten steel, the adoption of a combination of W-RHtreatment+high-temperature tapping at a temperature above theconventional operation, i.e., melting point+at least 100° C., theadoption of a combination of W-RH treatment+short LF, long RH treatmentwherein the operation time in the ladle furnace is shortened and, inaddition, the RH quantity of circulation in circulation degassing (thatis, amount of molten steel circulated/total amount of molten steelcirculated) is increased to satisfactorily perform degassing for a longperiod of time, and the adoption of a combination of all the abovetreatments, that is, a combination of the W-RHtreatment+high-temperature tapping+short LF, long RH, can realize, forboth steel types, SUJ 2 and SCM 435, lowered oxygen content of productsand significantly decreased number of inclusions having a size of notless than 20 μm. Further, as can be seen from Tables B1 to B8, for theexamples of the present invention, regarding the cleanliness, all thesteel products are evaluated as good (◯) and excellent (⊚), that is, areexcellent high-cleanliness steels. By contrast, as can be seen fromTables B9 and B10, for all the conventional examples, the cleanliness isevaluated as failure (X), and the conventional steel products cannot besaid to be clean steels.

[0169] For the heats wherein the W-RH treatment has been carried out,both the oxygen content and the predicted value of the maximum inclusiondiameter are reduced by increasing T_(SH) [(temperature at which moltensteel is transferred to ladle furnace)−(melting point of moltensteel)=T_(SH))] to improve the cleanliness. For heats in which the W-RHtreatment has been carried out, regarding the relationship of therefining time in the ladle furnace with the oxygen content and thepredicted value of the maximum inclusion diameter, when the refiningtime is not less than about 25 min, the oxygen content and the predictedvalue of the maximum inclusion diameter are satisfactorily lowered. Thepredicted value of the maximum inclusion diameter, however, increaseswith increasing the refining time. The reason for this is considered asfollows. With the elapse of time, the melt loss of refractories in theladle refining furnace is increased, the equilibrium of the slag systemis broken, for example, as a result of oxidation due to the contact withthe air, and the level of the dissolved oxygen goes beyond the minimumlevel of dissolved oxygen. Further, the relationship of the amount ofmolten steel circulated/total amount of molten steel in thecirculation-type vacuum degassing device with the oxygen content and thepredicted value of the maximum inclusion diameter, the effect ofenhancing the cleanliness increases with increasing the amount of moltensteel circulated, and is substantially saturated when the amount ofmolten steel circulated/total amount of molten steel is not less than 15times.

[0170] It was confirmed that reducing the oxygen content and thepredicted value of the maximum inclusion diameter results in improvedL₁₀ life. This indicates that steels produced by the process accordingto the present invention, which can reduce the oxygen content and thepredicted value of the maximum inclusion diameter, have excellentfatigue strength properties such as excellent rolling fatigue life.

[0171] FIG. B1 is a diagram showing the oxygen content of products in 10heats in the production process according to the present invention usingW-RH treatment wherein, in the treatment of molten steel for steel SUJ2, pre-degassing is performed before ladle refining and, in addition,after the ladle refining, the molten steel is degassed, and the oxygencontent of products in 10 heats in the conventional process wherein thepre-deoxidation is not carried out. In FIGS. B1, B3, and B5, A₁ showsdata on the adoption of only W-RH treatment according to the presentinvention defined in claim 8, A₂ data on the W-RHtreatment+high-temperature tapping according to the present inventiondefined in claim 9, A3 data on the W-RH treatment+short-time LF,long-time RH treatment according to the present invention defined inclaim 10, A₄ data on the W-RH treatment+high-temperaturetapping+short-time LF, long-time RH treatment according to the presentinvention defined in claim 10, and conventional data on prior artwherein the pre-degassing is not carried out.

[0172] FIG. B2 is a diagram showing the oxygen content of products in 10heats in the production process according to the present invention usingW-RH treatment wherein, in the treatment of molten steel for steel SCM435, pre-degassing is performed before ladle refining and, in addition,after the ladle refining, the molten steel is degassed, and the oxygencontent of products in 10 heats in the conventional process wherein thepre-deoxidation is not carried out. In FIGS. B2, B4, and B6, B₁ showsdata on the adoption of only W-RH treatment according to the presentinvention defined in claim 8, B2 data on the W-RHtreatment+high-temperature tapping according to the present inventiondefined in claim 9, B3 data on the W-RH treatment+short-time LF,long-time RH treatment according to the present invention defined inclaim 10, 14 data on the W-RH treatment+high-temperaturetapping+short-time LF, long-time RH treatment according to the presentinvention defined in claim 10, and conventional data on prior artwherein the pre-degassing is not carried out.

[0173] FIG. B3 is a diagram showing the maximum predicted inclusiondiameter determined according to statistics of extreme values ofproducts in 10 heats in the production process according to the presentinvention using W-RH treatment wherein, in the treatment of molten steelfor steel SUJ 2, pre-degassing is performed before ladle refining and,in addition, after the ladle refining, the molten steel is degassed, andthe maximum predicted inclusion diameter of products in 10 heats in theconventional process wherein the pre-degassing is not carried out.

[0174] FIG. B4 is a diagram showing the maximum predicted inclusiondiameter determined according to statistics of extreme values ofproducts in 10 heats in the production process according to the presentinvention using W-RH treatment wherein, in the treatment of molten steelfor steel SCM 435, pre-degassing is performed before ladle refining and,in addition, after the ladle refining, the molten steel is degassed, andthe maximum predicted inclusion diameter of products in 10 heats in theconventional process wherein the pre-degassing is not carried out.

[0175] FIG. B5 shows data on L₁₀ service life of products as determinedby a thrust rolling service life test in 10 heats in the productionprocess according to the present invention using W-RH treatment wherein,in the treatment of molten steel for steel SUJ 2, pre-degassing isperformed before ladle refining and, in addition, after the ladlerefining, the molten steel is degassed, and the L₁₀ service life ofproducts in 10 heats in the conventional process wherein thepre-degassing is not carried out.

[0176] FIG. B6 shows data on L₁₀ service life as determined by a thrustrolling service life test in 10 heats in the production processaccording to the present invention using W-RH treatment wherein, in thetreatment of molten steel for steel SCM 435, pre-degassing is performedbefore ladle refining and, in addition, after the ladle refining, themolten steel is degassed, and the L₁₀ service life of products in 10heats in the conventional process wherein the pre-degassing is notcarried out.

[0177] As is apparent from the test results, it was confirmed that, forboth steel SUJ 2 and steel SCM 435, W-RH treatment, whereinpre-degassing is performed before ladle refining and, in addition, afterthe ladle refining, the molten steel is degassed, can significantlyreduce both the oxygen content of the products and the predicted valueof the maximum inclusion diameter and, according to the process of thepresent invention, the cleanliness is significantly improved and the L₁₀life as determined by the thrust rolling service life test issignificantly improved. The addition of treatments to the process, thatis, the addition of only W-RH treatment according to the presentinvention as defined in claim 8, the addition of W-RHtreatment+high-temperature tapping according to the present inventiondefined in claim 9, and the addition of W-RH treatment+short-time LF,long-time RH treatment or the addition of W-RHtreatment+high-temperature tapping+short-time LF, long-time RH treatmentaccording to the present invention defined in claim 10, cansignificantly improve all the oxygen content of products, the predictedvalue of the maximum inclusion diameter, and the L₁₀ life as determinedby the thrust rolling service life test.

[0178] As is apparent from the foregoing description, according to thepresent invention, a large quantity of steel products having a very highlevel of cleanliness can be provided without use of a remelting processwhich incurs very high cost. This can realize the provision ofhigh-cleanliness steels for use as steels for mechanical parts requiredto possess fatigue strength and fatigue life, particularly, for example,as steels for rolling bearings, steels for constant velocity joints,steels for gears, steels for continuously variable transmission oftoroidal type, steels for mechanical structures for cold forging, toolsteels, and spring steels, and processes for producing the same, thatis, can offer unprecedented excellent effect.

EXAMPLE C

[0179] A molten steel was subjected to oxidizing refining in an arcmelting furnace. In the same furnace, deoxidizers, such as aluminum andsilicon, were then added to the refined molten steel to deoxidize themolten steel. The pre-deoxidized molten steel was transferred to a ladlefurnace to perform ladle refining. The refined molten steel was thendegassed in a circulation-type vacuum degassing device, followed by aningot production process using casting. Steel products of JIS SUJ 2 andSCM 435 in 10 heats thus obtained were examined for the oxygen contentof the products, the predicted value of the maximum inclusion diameteraccording to statistics of extreme values, and L₁₀ service life by athrust-type rolling service lift test. In the measurement of thepredicted value of the maximum inclusion diameter, a test piece wastaken off from a φ65 forged material, the observation of 100 mm² wascarried out for 30 test pieces, and the maximum inclusion diameter in30000 mm² was predicted according to statistics of extreme values. Inthe thrust-type rolling service life test, a test piece having a size ofφ60×φ20×8.3T, which had been subjected to carburizing, quench hardeningand tempering, was tested at a maximum hertz stress Pmax: 4900 MPa,followed by calculation to determine the L₁₀ service life.

[0180] An example of the operation of oxidizing refining in an arcmelting furnace or a converter followed by deoxidation in the samefurnace (hereinafter referred to as “in-furnace deoxidation”), that is,only in-furnace deoxidation, according to the present invention for 10heats of steel SUJ 2 is shown in Table C1. TABLE C1 Operation In-furnacedeoxidation (A₁) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Amount of deoxidizer (Si,Mn, Al, 3.7 2 4.6 4.3 3.6 5 5.9 4.9 4.4 4.9 etc.) added in in-furnacedeoxidation, kg/t Tapping temp.: m.p. + ° C. 59 67 70 52 55 71 69 69 5869 LF: Time, min 59 57 53 54 57 57 54 58 53 53 LF: Termination temp., °C. 1524 1520 1520 1526 1520 1520 1524 1521 1525 1521 RH: Time, min 23 2323 23 23 23 23 23 23 23 RH: Quantity of circulation, times 7.1 6.3 7 6.17.1 6.8 6.7 5.9 6.7 7.2 RH: Termination temp., ° C. 1497 1499 1500 14941500 1494 1496 1498 1496 1499 Casting temp., ° C. 1478 1475 1477 14771475 1475 1476 1475 1475 1475 Oxygen content of product, ppm 4.8 5.2 55.6 4.6 4.8 4.6 5.7 5 5 Number of inclusions of not less 29 40 32 25 3026 37 27 27 34 than 20 μm in 100 g of steel product Maximum predicteddiameter of 48 41.6 50 56 36.8 43.2 41.4 51.3 50 50 inclusions, μm L₁₀(× 10⁷) 2.5 1.9 2.4 2.6 2.1 2.7 2.2 1.8 2.2 1.8 Results of evaluation ΔΔ Δ Δ Δ Δ Δ Δ Δ Δ

[0181] An example of the operation of only in-furnace deoxidationaccording to the present invention for 10 heats of steel SCM 435 isshown in Table C2. TABLE C2 Operation In-furnace deoxidation (B₁) No. 12 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Amount of deoxidizer(Si, 5.4 5.72.3 2.7 4.7 2.5 5.1 5.3 5.4 5.1 Mn, Al, etc.) added in in- furnacedeoxidation, kg/t Tapping temp.: m.p. + ° C. 60 65 66 54 63 64 57 61 6051 LF: Time, min 60 54 54 52 58 52 54 56 57 56 LF: Termination temp., °C. 1575 1572 1570 1570 1565 1572 1568 1566 1567 1572 RH: Time, min 20 2020 24 21 23 21 20 21 23 RH: Quantity of circulation, 6.7 6.2 6.5 6.6 6.37.3 7.1 6.9 5.7 5.8 times RH: Termination temp., ° C. 1540 1540 15351534 1541 1539 1541 1536 1536 1533 Casting temp., ° C. 1520 1517 15211518 1515 1519 1520 1520 1514 1520 Oxygen content of product, 8.5 8.38.1 7.1 7.0 7.3 8.0 8.1 6.7 6.9 ppm Number of inclusions of not 35 28 2532 29 27 37 32 38 33 less than 20 μm in 100 g of steel product Maximumpredicted diameter 51.0 58.1 48.6 49.7 42.0 51.1 56.0 48.6 40.2 48.3 ofinclusions, μm L₁₀ (× 10⁷) 1.5 1.8 2.1 1.8 2.3 1.7 1.6 2.5 2.2 2.3Results of evaluation Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ

[0182] An example of the operation of in-furnacedeoxidation+high-temperature tapping according to the present inventionfor 10 heats of steel SUJ 2 is shown in Table C3. TABLE C3 OperationIn-furnace deoxidation + tapping temp. (A₂) No. 1 2 3 4 5 6 7 8 9 10Type of steel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ2 Amount of deoxidizer (Si, Mn, Al, 3.1 2 3.2 4.6 2 4.8 2.1 3 3.3 4.1etc.) added in in-furnace deoxidation, kg/t Tapping temp.: m.p. + ° C.187 178 124 143 178 142 175 163 180 142 LF: Time, min 54 59 57 59 60 6057 59 56 54 LF: Termination temp., ° C. 1523 1525 1522 1526 1525 15201524 1525 1522 1520 RH: Time, min 23 23 23 23 23 23 23 23 23 23 RH:Quantity of circulation, times 7.2 6.1 6.3 7 6.7 5.5 6.4 5.9 5.8 6 RH:Termination temp., ° C. 1501 1503 1500 1499 1496 1496 1498 1493 14921499 Casting temp., ° C. 1477 1476 1478 1475 1475 1475 1475 1478 14761478 Oxygen content of product, ppm 4.8 4.5 4.6 4.6 4.7 5.1 4.6 4.9 4.94.7 Number of inclusions of not less 19 19 19 18 26 30 24 22 30 24 than20 μm in 100 g of steel product Maximum predicted diameter of 19.2 22.518.4 23 23.5 25.5 18.4 19.6 24.5 18.8 inclusions, μm L₁₀ (× 10⁷) 4.0 3.84.4 3.9 4.3 4.3 3.9 4.1 3.7 3.7 Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯◯

[0183] An example of the operation of in-furnacedeoxidation+high-temperature tapping according to the present inventionfor 10 heats of steel SCM 435 is shown in Table C4. TABLE C4 OperationIn-furnace deoxidation + tapping temp. (B₂) No. 1 2 3 4 5 6 7 8 9 10Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 SCM 435 Amount of deoxidizer (Si, 5.2 5 6 6 1.9 5.8 4.84.8 3.4 2.7 Mn, Al, etc.) added in in- furnace deoxidation, kg/t Tappingtemp.: m.p. + ° C. 124 140 123 109 112 117 123 116 104 143 LF: Time, min54 45 55 49 48 52 48 45 45 54 LF: Termination temp., ° C. 1567 1566 15731575 1575 1572 1566 1565 1567 1567 RH: Time, min 22 24 22 24 20 21 24 2123 24 RH: Quantity of circulation, 7.2 6.5 5.6 6.8 6.7 5.9 6.4 7.2 6.36.5 times RH: Termination temp., 1535 1539 1532 1538 1538 1536 1538 15331541 1541 ° C. Casting temp., ° C. 1513 1513 1520 1514 1518 1521 15211521 1518 1518 Oxygen content of product, 7.2 6.8 7.0 7.0 6.4 6.8 7.57.3 6.5 6.1 ppm Number of inclusions of not 30 16 19 23 29 30 30 21 2526 less than 20 μm in 100 g of steel product Maximum predicted diameter39.0 38.1 37.1 38.5 37.8 39.8 39.0 39.4 33.8 32.9 of inclusions, μm L₁₀(× 10⁷) 2.8 3.3 2.9 3.5 3.1 3.5 3.3 3.0 3.7 3.6 Results of evaluation ◯◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0184] An example of the operation of in-furnace deoxidation+short LF,long RH according to the present invention for 10 heats of steel SUJ 2is shown in Table C5. TABLE C5 Operation In-furnace deoxidation + shortLF, long RH (A₃) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Amount of deoxidizer (Si,Mn, Al, 4.7 5 4.4 2.3 2.6 2 4.5 2.3 3.6 4.5 etc.) added in in-furnacedeoxidation, kg/t Tapping temp.: m.p. + ° C. 67 79 59 78 64 72 75 75 6972 LF: Time, min 43 31 45 40 37 35 41 30 37 45 LF: Termination temp., °C. 1546 1543 1545 1544 1545 1541 1544 1545 1546 1545 RH: Time, min 53 5656 59 59 59 60 56 56 58 RH: Quantity of circulation, times 17.7 18.718.7 19.7 19.7 19.7 20.0 18.7 18.7 19.3 RH: Termination temp., ° C. 15081502 1508 1510 1505 1508 1509 1508 1506 1506 Casting temp., ° C. 14761477 1477 1478 1478 1478 1475 1477 1478 1475 Oxygen content of product,ppm 4.9 4.4 4.6 4.5 4.1 5.1 5 4.3 5 5.1 Number of inclusions of not less29 27 27 25 26 29 29 22 20 24 than 20 μm in 100 g of steel productMaximum predicted diameter of 18 18 22.8 21.1 20.8 20.5 18.2 20.6 22.618.7 inclusions, μm L₁₀ (× 10⁷) 5.7 5.9 5.1 5.4 5.7 5.5 5.8 5.6 5.2 6.0Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0185] An example of the operation of in-furnace deoxidation+short LP,long RH according to the present invention for 10 heats of steel SCM 435is shown in Table C6. TABLE C6 Operation In-furnace deoxidation + shortLF, Long RH (B₃) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Amountof deoxidizer (Si, 3.9 4.4 2.7 4.5 3.6 3 2.6 2.5 2.2 5.8 Mn, Al, etc.)added in in- furnace deoxidation, kg/t Tapping temp.: m.p. + ° C. 66 6256 71 58 70 80 75 62 62 LF: Time, min 41 44 44 44 42 39 44 39 43 38 LF:Termination temp., ° C. 1581 1577 1584 1582 1577 1578 1579 1583 15831578 RH: Time, min 39 41 37 43 43 44 38 37 38 45 RH: Quantity ofcirculation, 13.0 13.7 12.3 14.3 14.3 14.7 12.7 12.3 12.7 15.0 times RH:Termination temp., ° C. 1540 1534 1536 1534 1539 1532 1537 1533 15401533 Casting temp., ° C. 1513 1513 1516 1514 1514 1515 1514 1514 15151514 Oxygen content of product, 7 7.1 7.3 7.4 7.3 6.5 7 6.9 6.9 6.7 ppmNumber of inclusions of not 25 28 25 25 24 23 24 25 26 23 less than 20μm in 100 g of steel product Maximum predicted diameter 23.7 20.7 24.622.7 22.9 23.7 22.8 21.7 24.8 24.6 of inclusions, μm L₁₀ (× 10⁷) 4.5 5.14.4 4.8 4.9 5.1 4.8 4.8 4.3 5.7 Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯◯

[0186] An example of the operation of in-furnacedeoxidation+high-temperature tapping+short LF, long RH according to thepresent invention for 10 heats of steel SUJ 2 is shown in Table C7.TABLE C7 Operation In-furnace deoxidation + tapping temp. + short LF,Long RH (A₄) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ 2SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Amount of deoxidizer (Si, Mn,Al, 2.8 2.4 3.6 5.6 3.1 1.5 2.1 5.9 3.1 1.6 etc.) added in in-furnacedeoxidation, kg/t Tapping temp.: m.p. + ° C. 133 149 162 164 119 138 122163 137 143 LF: Time Min 39 36 36 42 43 37 38 30 42 37 LF: Terminationtemp., ° C. 1546 1543 1545 1544 1545 1541 1544 1545 1546 1545 RH: Time,min 53 53 53 53 56 52 57 53 52 56 RH: Quantity of circulation, times17.7 18.3 17.8 17.1 18.7 17.9 18.4 17.5 16.7 19.3 RH: Termination temp.,° C. 1495 1497 1503 1502 1501 1503 1497 1503 1500 1503 Casting temp., °C. 1475 1476 1476 1477 1475 1478 1476 1477 1478 1477 Oxygen content ofproduct, ppm 4.8 4.2 4.7 4.7 4.4 4.1 4.4 4.8 4.5 4.2 Number ofinclusions of not less 14 6 8 9 6 14 13 8 15 14 than20 μm in100 g ofsteel product Maximum predicted diameter of 14.3 13.6 14.1 14.8 13.213.7 13.2 14.4 14.8 12.6 inclusions, μm L₁₀ (× 10⁷) 7.8 9.0 8.7 8.7 10.69.7 10.8 9.4 9.8 10.0 Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0187] An example of the operation of in-furnacedeoxidation+high-temperature tapping+short LF, long RH according to thepresent invention for 10 heats of steel SCM 435 is shown in Table C8.TABLE C8 Operation In-furnace deoxidation + tapping temp. + short LF,long RH (B₄) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Amount ofdeoxidizer (Si, 4.3 4 1.7 2.2 4.1 2.3 4.5 4.6 1.5 2.1 Mn, Al, etc.)added in in- furnace deoxidation, kg/t Tapping temp.: m.p. + ° C. 134132 117 107 132 137 128 109 116 102 LF: Time, min 39 33 30 41 30 36 3235 35 44 LF: Termination temp., ° C. 1577 1581 1577 1585 1584 1582 15821576 1582 1584 RH: Time, min 39 39 36 42 38 42 38 40 39 41 RH: Quantityof circulation, 11.9 12.7 12.1 13.1 11.0 14.0 11.7 12.2 12.3 12.7 timesRH: Termination temp., ° C. 1534 1540 1534 1540 1541 1532 1539 1531 15381532 Casting temp., ° C. 1512 1513 1516 1513 1513 1515 1512 1516 15141518 Oxygen content of product, 6.3 5.5 5.5 5.4 6.0 6.0 5.6 6.5 5.7 5.6ppm Number of inclusions of not 13 6 11 9 5 8 11 14 10 14 less than 20μm in 100 g of steel product Maximum predicted diameter 24.0 23.5 23.322.5 23.9 23.7 23.8 24.6 23.7 23.6 of inclusions, μm L₁₀ (× 10⁷) 9.2 8.810.1 9.7 10.3 8.7 9.8 9.9 10.7 9.9 Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚⊚ ⊚

[0188] For comparison with the present invention, an example of theoperation according to a prior art technique for steel SUJ 2 is shown inTable C9, and an example of the operation according to a prior arttechnique for SCM 435 is shown in Table C10. TABLE C9 OperationConventional operation (prior art) No. 1 2 3 4 5 6 7 8 9 10 Type ofsteel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Amountof deoxidizer (Si, Mn, Al, 57 72 58 60 74 75 51 65 62 68 etc.) added inin-furnace deoxidation, kg/t Tapping temp.: m.p. + ° C. — — — — — — — —— — LF: Time, min 61 61 63 61 62 62 61 63 61 63 LF: Termination temp., °C. 1525 1524 1526 1525 1523 1524 1523 1520 1525 1520 RH: Time, min 23 2323 23 23 23 23 23 23 23 RH: Quantity of circulation, times 5.7 6.7 7.16.5 6.2 5.7 7 5.5 6.8 6.2 RH: Termination temp., ° C. 1493 1502 15011497 1501 1501 1502 1503 1496 1499 Casting temp., ° C. 1477 1475 14751475 1475 1475 1476 1478 1478 1476 Oxygen content of product, ppm 5.45.1 5.1 6.1 5.8 5.9 5.8 5.9 5.2 6.2 Number of inclusions of not less 5956 54 65 48 41 50 47 45 49 than 20 μm in 100 g of steel product Maximumpredicted diameter of 86.4 61.2 66.3 97.6 81.2 76.7 92.8 76.7 72.8 74.4inclusions, μm L₁₀ (× 10⁷) 1.9 2.4 2.4 1.8 1.9 3.4 1.9 2.2 2.0 2.2Results of evaluation X X X X X X X X X X

[0189] TABLE C10 Operation Conventional operation (prior art) No. 1 2 34 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM435 SCM 435 SCM 435 SCM 435 SCM 435 Amount of deoxidizer (Si, 61 54 6950 74 58 58 69 64 54 Mn, Al, etc.) added in in- furnace deoxidation,kg/t Tapping temp.: m.p. + ° C. — — — — — — — — — — LF: Time, min 62 6361 61 61 63 63 63 61 61 LF: Termination temp., ° C. 1570 1574 1566 15721567 1569 1567 1569 1569 1570 RH: Time, min 23 23 23 20 21 23 21 23 2324 RH: Quantity of circulation, 6.8 7.5 7.0 8.3 6.2 6.0 7.4 8.0 7.3 6.7times RH: Termination temp., ° C. 1533 1538 1541 1540 1541 1533 15351534 1531 1531 Casting temp., ° C. 1517 1519 1520 1518 1517 1511 15161512 1512 1521 Oxygen content of product, 7.6 9.2 9.2 8.8 6.9 8.3 6.98.3 9.4 9.1 ppm Number of inclusions of not 49 54 59 52 42 57 56 53 5342 less than 20 μm in 100 g of steel product Maximum predicted diameter68.4 82.8 73.6 70.4 55.2 83.0 55.2 83.0 84.6 91.0 of inclusions μm L₁₀(× 10⁷) 1.0 1.3 1.1 1.9 2.3 1.5 2.0 1.2 1.2 1.9 Results of evaluation XX X X X X X X X X

[0190] As is apparent from Tables C1 to C8, for steel products producedaccording to the present invention wherein a molten steel produced in anarc melting furnace or a converter is subjected to in-furnacedeoxidation in the same furnace, is transferred to a ladle furnace toperform refining, and is then circulated through a circulation-typevacuum degassing device to degas the molten steel, for steels producedusing a combination of in-furnace deoxidation+high-temperature tappingat a temperature above the conventional operation, i.e., meltingpoint+at least 100° C., for steels produced using a combination ofin-furnace deoxidation+short LF, long RH treatment wherein the operationtime in the ladle furnace is shortened and, in addition, the RH quantityof circulation in circulation degassing (that is, amount of molten steelcirculated/total amount of molten steel circulated) is increased tosatisfactorily perform degassing for a long period of time, and forsteels produced using a combination of all the above treatments, thatis, a combination of the in-furnace deoxidation+high-temperaturetapping+short LF, long RH, can realize, for both steel types, SUJ 2 andSCM 435, lowered oxygen content of products and significantly decreasednumber of inclusions having a size of not less than 20 μm. Further, ascan be seen from Tables C1 to C8, for the examples of the presentinvention, regarding the cleanliness, all the steel products areevaluated as fair (Δ), good (◯), or excellent (⊚), that is, areexcellent high-cleanliness steels. By contrast, as can be seen fromTables C9 and C10, for all the conventional examples, the cleanliness isevaluated as failure (X), and the conventional steel products cannot besaid to be clean steels. In this connection, it should be noted thatfair (Δ) is based on the comparison with good (◯) and excellent (⊚) and,as compared with steels produced according to the conventional processinvolving no tapping deoxidation which is evaluated as failure (X), thesteels evaluated as fair (Δ) have much higher cleanliness.

[0191] For the heats wherein the in-furnace deoxidation has been carriedout, both the oxygen content and the predicted value of the maximuminclusion diameter are reduced by increasing T_(SH) [(temperature atwhich molten steel is transferred to ladle refining furnace)−(meltingpoint of molten steel)=T_(SH))] to improve the cleanliness. For theheats in which the in-furnace deoxidation has been carried out,regarding the relationship of the refining time in the ladle furnacewith the oxygen content and the predicted value of the maximum inclusiondiameter, when the refining time is not less than about 25 min, theoxygen content and the predicted value of the maximum inclusion diameterare satisfactorily lowered. The predicted value of the maximum inclusiondiameter, however, increases with increasing the refining time. Thereason for this is considered as follows. With the elapse of time, themelt loss of refractories in the ladle furnace is increased, theequilibrium of the slag system is broken, for example, as a result ofoxidation due to the contact with the air, and the level of thedissolved oxygen goes beyond the minimum level of dissolved oxygen.Further, the relationship of the amount of molten steel circulated/totalamount of molten steel in the circulation-type vacuum degassing devicewith the oxygen content and the predicted value of the maximum inclusiondiameter, the effect of enhancing the cleanliness increases withincreasing the amount of molten steel circulated, and is substantiallysaturated when the amount of molten steel circulated/total amount ofmolten steel is not less than 15 times.

[0192] It was confirmed that reducing the oxygen content and thepredicted value of the maximum inclusion diameter results in improvedL₁₀ life. This indicates that steels produced by the process accordingto the present invention, which can reduce the oxygen content and thepredicted value of the maximum inclusion diameter, have excellentfatigue strength properties such as excellent rolling fatigue life.

[0193] FIG. C1 is a diagram showing the oxygen content of products in 10heats in the production process according to the present inventionwherein, in the treatment of a molten steel for steel SUJ 2, a moltensteel is subjected to oxidizing refining in an arc melting furnace or aconverter, a deoxidizer is then added to the same furnace before tappingto deoxidize the molten steel, and the deoxidized molten steel istransferred to a ladle furnace to perform ladle refining, and is thencirculated through a circulation-type vacuum degassing device to degasthe molten steel, and the oxygen content of products in 10 heats in theconventional process wherein the in-furnace deoxidation is not carriedout. In FIGS. C1, C3, and C5, A₁ shows data on the adoption of onlyin-furnace deoxidation according to the present invention defined inclaim 15, A₂ data on in-furnace deoxidation+high-temperature tappingaccording to the present invention defined in claim 16, A₃ data onin-furnace deoxidation+short-time LF, long-time RH treatment accordingto the present invention defined in claim 17, A₄ data on in-furnacedeoxidation+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention defined in claim 17, andconventional data on prior art.

[0194] FIG. C2 is a diagram showing the oxygen content of products in 10heats in the production process according to the present inventionwherein, in the treatment of a molten steel for steel SCM 435, a moltensteel is subjected to oxidizing refining in an arc melting furnace or aconverter, a deoxidizer is then added to the same furnace before tappingto dioxidize the molten steel, and the deoxidized molten steel istransferred to a ladle furnace to perform ladle refining, and is thencirculated through a circulation-type vacuum degassing device to degasthe molten steel, and the oxygen content of products in 10 heats in theconventional process wherein the in-furnace deoxidation is not carriedout. In FIGS. 16, 18, and 20, B₁ shows data on the adoption of onlyin-furnace deoxidation according to the present invention defined inclaim 15, B₂ data on in-furnace deoxidation+high-temperature tappingaccording to the present invention defined in claim 16, B₃ data onin-furnace deoxidation+short-time LF, long-time RH treatment accordingto the present invention defined in claim 17, B₄ data on in-furnacedeoxidation+high-temperature tapping+short-time LF, long-time RHtreatment according to the present invention defined in claim 17, andconventional data on the conventional process wherein the in-furnacedeoxidation is not carried out.

[0195] FIG. C3 is a diagram showing the maximum predicted inclusiondiameter of products determined according to statistics of extremevalues in 10 heats in the production process of the present inventionusing in-furnace deoxidation in the treatment of a molten steel forsteel SUJ 2 according to claims 15 to 17, and the maximum predictedinclusion diameter of products in 10 heats in the conventional processwherein the in-furnace deoxidation is not carried out.

[0196] FIG. C4 is a diagram showing the maximum predicted inclusiondiameter of products determined according to statistics of extremevalues in 10 heats in the production process of the present inventionusing in-furnace deoxidation in the treatment of a molten steel forsteel SCM 435 according to claims 15 to 17, and the maximum predictedinclusion diameter of products in 10 heats in the conventional processwherein the in-furnace deoxidation is not carried out.

[0197] FIG. C5 shows data on L₁₀ service life of products as determinedby a thrust rolling service life test in 10 heats in the productionprocess of the present invention using in-furnace deoxidation in thetreatment of a molten steel for steel SUJ 2 according to claims 15 to17, and the L₁₀ service life of products in 10 heats in the conventionalprocess wherein the in-furnace deoxidation is not carried out.

[0198] FIG. C6 shows data on L₁₀ service life of products as determinedby a thrust rolling service life test in 10 heats in the productionprocess of the present invention using in-furnace deoxidation in thetreatment of a molten steel for steel SCM 435 according to claims 15 to17, and the L₁₀ service life of products in 10 heats in the conventionalprocess wherein the in-furnace deoxidation is not carried out.

[0199] As is apparent from the test results, it was confirmed that, forboth steel SUJ 2 and steel SCM 435, the adoption of a method wherein amolten steel is subjected to oxidizing refining in an arc meltingfurnace or a converter, a deoxidizer is then added to the same furnacebefore tapping to deoxidize the molten steel, and the deoxidized moltensteel is transferred to a ladle furnace to perform ladle refining, andis then circulated through a circulation-type vacuum degassing device todegas the molten steel, can significantly reduce both the oxygen contentof the products and the predicted value of the maximum inclusiondiameter and, according to the process of the present invention, thecleanliness is significantly improved and the L₁₀ life as determined bythe thrust rolling service life test is significantly improved. Theaddition of treatments to the process, that is, the addition of onlyin-furnace deoxidation according to the present invention as defined inclaim 15, the addition of in-furnace deoxidation+high-temperaturetapping according to the present invention defined in claim 16, and theaddition of in-furnace deoxidation+short-time LF, long-time RH treatmentaccording to the present invention as defined in claim 17 or theaddition of in-furnace deoxidation+high-temperature tapping+short-timeLF, long-time RH treatment according to the present invention defined inclaim 17, can significantly improve all the oxygen content of products,the predicted value of the maximum inclusion diameter, and the L₁₀ lifeas determined by the thrust rolling service life test.

[0200] As is apparent from the foregoing description, according to thepresent invention, a large quantity of steel products having a very highlevel of cleanliness can be provided without use of a remelting processwhich incurs very high cost. This can realize the provision ofhigh-cleanliness steels for use as steels for mechanical parts requiredto possess fatigue strength and fatigue life, particularly, for example,as steels for rolling bearings, steels for constant velocity joints,steels for gears, and steels for continuously variable transmission oftoroidal type, that is, can offer unprecedented excellent effect.

EXAMPLE D

[0201] A molten steel, which had been subjected to oxidizing smeltingand produced by a melting process in an arc melting furnace was thentransferred to a ladle furnace where the molten steel was subjected toladle refining for a short period of time of not more than 60 min. Next,degassing was carried out for not less than 25 min. In particular,degassing was carried out in a circulation-type vacuum degassing devicein such a manner that the amount of the molten steel circulated was notless than 8 times the total amount of the molten steel, followed by aningot production process using casting. Steel products of JIS SUJ 2 andSCM 435 in 10 heats thus obtained were examined for the oxygen contentof the products, the predicted value of the maximum inclusion diameteraccording to statistics of extreme values, and L₁₀ service life by athrust-type rolling service life test. In the measurement of thepredicted value of the maximum inclusion diameter, a test piece wastaken off from a φ65 forged material, the observation of 100 mm² wascarried out for 30 test pieces, and the maximum inclusion diameter in30000 mm² was predicted according to statistics of extreme values. Inthe thrust-type rolling service life test, a test piece having a size ofφ60×φ20×8.3T, which had been subjected to carburizing, quench hardeningand tempering, was tested at a maximum hertz stress Pmax: 4900 MPa,followed by calculation to determine the L₁₀ service life.

[0202] An example of the operation of oxidizing refining in an arcmelting furnace or a converter followed by the transfer of the moltensteel to a ladle furnace where the ladle refining-was carried out fornot more than 60 min and degassing was then carried out in acirculation-type vacuum degassing device for not less than 25 min (herethis being referred to as “short-time LF, long-time RH or short LF orlong RH”), that is, short-time LF, long-time RH, for 10 heats of steelSUJ 2 is shown in Table D1. TABLE D1 Operation Short LF, long RH (A₁)No. 1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. + ° C. 67 79 59 78 64 7275 61 57 59 LF: Time, min 43 31 45 40 37 35 41 30 37 45 LF: Terminationtemp., ° C. 1546 1543 1545 1544 1526 1541 1544 1534 1530 1524 RH: Time,min 53 56 56 59 29 59 60 44 38 27 RH: Quantity of circulation, times17.7 18.7 18.7 19.7 9.0 19.7 20.0 13.7 11.9 8.5 RH: Termination temp., °C. 1508 1502 1508 1510 1505 1508 1509 1508 1506 1506 Casting temp., ° C.1476 1477 1477 1478 1478 1478 1475 1477 1478 1475 Oxygen content ofproduct, ppm 4.9 4.4 4.6 4.5 5.3 5.1 5 4.8 5.2 5 Number of inclusions ofnot less 29 27 27 25 30 29 29 26 27 28 than 20 μm in 100 g of steelproduct Maximum predicted diameter of 18 18 22.8 21.1 22.9 20.5 18.220.6 20.1 21.7 inclusions, μm L₁₀ (× 10⁷) 5.7 5.1 4.1 4.9 4.6 4.1 5.34.2 4.7 4.7 Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯

[0203] An example of the operation of oxidizing μmelting in an arcmelting furnace or a converter followed by the transfer of the moltensteel to a ladle furnace where the ladle refining was carried out fornot more than 60 min and degassing was then carried out in acirculation-type vacuum degassing device for not less than 25 min, thatis, short-time LF, long-time RH treatment, for 10 heats of steel SCM 435is shown in Table D2. TABLE D2 Operation Short LF, long RH (B₁) No. 1 23 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Tapping temp.: m.p. + ° C. 66 6256 71 58 70 80 75 62 62 LF: Time, min 41 44 44 44 42 39 44 39 43 38 LF:Termination temp., ° C. 1581 1568 1584 1571 1577 1578 1579 1583 15721578 RH: Time, min 39 26 37 30 43 44 38 37 29 45 RH: Quantity ofcirculation, 13.0 8.2 12.3 9.5 14.3 14.7 12.7 12.3 8.8 15.0 times RH:Termination temp., ° C. 1540 1534 1536 1534 1539 1532 1537 1533 15401533 Casting temp., ° C. 1513 1513 1516 1514 1514 1515 1514 1514 15151514 Oxygen content of product, 7 7.7 7.3 7.5 7.3 6.5 7 6.9 7.4 6.7 ppmNumber of inclusions of not 25 29 25 27 24 23 24 25 28 23 less than 20μm in 100 g of steel product Maximum predicted diameter 23.7 24.8 24.624.1 22.9 23.7 22.8 21.7 24.2 24.6 of inclusions, μm L₁₀ (× 10⁷) 2.9 2.33.9 3.4 3.4 3.5 3.8 4.0 3.0 3.9 Results of evaluation ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯◯

[0204] An example of the operation of oxidizing refining in an arcmelting furnace or a converter followed by tapping at a high temperatureof at least 100° C. above the melting point of the molten steel (in thisspecification, this being referred to as “high-temperature tapping”) toa ladle furnace where the ladle refining was carried out for not morethan 60 min and degassing was then carried out in a circulation-typevacuum degassing device for not less than 25 min, that is, short-timeLF, long-time RH treatment+high-temperature tapping, for 10 heats ofsteel SUJ 2 is shown in Table D3. TABLE D3 Operation Tapping temp. +short LF, long RH (A₂) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SUJ 2 SUJ2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 Tapping temp.: m.p. +° C. 133 149 162 164 119 138 122 163 137 143 LF: Time, min 39 36 36 4243 37 38 30 42 37 LF: Termination temp., ° C. 1531 1543 1545 1537 15451541 1544 1533 1524 1531 RH: Time, min 41 53 53 48 56 52 57 38 29 35 RH:Quantity of circulation, times 12.6 18.3 17.8 15.7 18.7 17.9 18.4 11.59.0 10.5 RH: Termination temp., ° C. 1495 1497 1503 1502 1501 1503 14971503 1500 1503 Casting temp., ° C. 1475 1476 1476 1477 1475 1478 14761477 1478 1477 Oxygen content of product, ppm 4.8 4.2 4.7 4.7 4.4 4.14.4 4.8 4.5 4.2 Number of inclusions of not less 14 6 8 9 6 14 13 8 1514 than 20 μm in 100 g of steel product Maximum predicted diameter of14.3 13.6 14.1 14.8 13.2 13.7 13.2 14.4 14.8 12.6 inclusions, μm L₁₀ (×10⁷) 8.0 10.6 9.6 8.8 9.0 9.4 9.7 7.3 7.7 10.9 Results of evaluation ⊚ ⊚⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0205] An example of the operation of oxidizing refining in an arcmelting furnace or a converter followed by tapping at a high temperatureof at least 100° C. above the melting point of the molten steel to aladle furnace where the ladle refining was carried out for not more than60 min and degassing was then carried out in a circulation-type vacuumdegassing device for not less than 25 min, that is, short-time LF,long-time RH treatment+high-temperature tapping, for 10 heats of steelSCM 435 is shown in Table D4. TABLE D4 Operation Tapping temp. + shortLF, long RH (B₂) No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 Tappingtemp.: m.p. + ° C. 134 132 117 107 132 137 128 109 116 102 LF: Time, min39 33 30 41 30 36 32 35 35 44 LF: Termination temp., ° C. 1577 1581 15771585 1584 1582 1582 1576 1570 1569 RH: Time, min 39 39 36 42 38 42 38 3328 29 RH: Quantity of circulation, 11.9 12.7 12.1 13.1 11.0 14.0 11.711.0 8.9 9.6 times RH: Termination temp., ° C. 1534 1540 1534 1540 15411532 1539 1531 1538 1532 Casting temp., ° C. 1512 1513 1516 1513 15131515 1512 1516 1514 1518 Oxygen content of product, 6.3 5.5 5.5 5.4 6.06.0 5.6 6.5 6.8 6.3 ppm Number of inclusions of not 13 6 11 9 5 8 11 1414 14 less than 20 μm in 100 g of steel product Maximum predicteddiameter 24.0 23.5 23.3 22.5 23.9 23.7 23.8 24.6 23.7 23.6 ofinclusions, μm L₁₀ (× 10⁷) 7.2 9.9 10.0 8.7 7.4 8.1 8.6 9.7 9.3 9.3Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0206] For comparison with the present invention, an example of theoperation according to a prior art technique for steel SUJ 2 is shown inTable D5, and an example of the operation according to a prior arttechnique for steel SCM 435 is shown in Table D6. TABLE D5 OperationConventional operation (prior art) No. 1 2 3 4 5 6 7 8 9 10 Type ofsteel SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2 SUJ 2Tapping temp.: m.p. + ° C. 70 70 79 58 77 76 73 55 58 60 LF: Time, min74 74 68 75 64 71 66 70 65 74 LF: Termination temp., ° C. 1523 1524 15241524 1523 1520 1522 1520 1523 1524 RH: Time, min 20 21 21 21 20 18 20 1923 22 RH: Quantity of circulation, times 6.7 7.0 7.0 7.0 6.7 6.0 6.7 6.37.7 7.3 RH: Termination temp., ° C. 1494 1497 1492 1493 1498 1498 14921499 1497 1499 Casting temp., ° C. 1476 1477 1478 1476 1475 1478 14781478 1475 1476 Oxygen content of product, ppm 5.7 5.7 5.8 5.2 6 5.1 5.35.2 5.6 6.3 Number of inclusions of not less 47 44 42 54 46 53 44 45 4443 than 20 μm in 100 g of steel product Maximum predicted diameter of76.3 77.2 68.2 68.5 82.3 63.9 76.5 91.3 70.3 68.5 inclusions, μm L₁₀ (×10⁷) 3.5 2.4 1.8 2.7 2.9 3.8 4.1 3.1 2.4 1.8 Results of evaluation X X XX X X X X X X

[0207] TABLE D6 Operation Conventional operation (prior art) No. 1 2 3 45 6 7 8 9 10 Type of steel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM435 SCM 435 SCM 435 SCM 435 SCM 435 Tapping temp.: m.p. + ° C. 61 62 6061 56 57 63 62 62 63 LF: Time, min 63 64 66 64 68 67 71 62 75 69 LF:Termination temp., ° C. 1565 1567 1569 1572 1565 1569 1566 1566 15651571 RH: Time, min 19 19 18 21 18 23 19 20 18 20 RH: Quantity ofcirculation, 6.3 6.3 6.0 7.0 6.0 7.7 6.3 6.7 6.0 6.7 times RH:Termination temp., ° C. 1535 1534 1536 1532 1541 1540 1535 1541 15391535 Casting temp., ° C. 1516 1519 1511 1518 1515 1516 1515 1517 15151512 Oxygen content of product, 9.5 6.5 5.3 5.5 6 6.3 6.3 6.3 5.7 5.2ppm Number of inclusions of not 51 49 48 58 60 43 56 47 43 54 less than20 μm in 100 g of steel product Maximum predicted diameter 58.3 60.465.8 72.6 69.7 75.3 78.7 61 78.6 83.9 of inclusions, μm L₁₀ (× 10⁷) 0.91.8 2.3 1.1 1.7 1.4 1.4 2.4 2.3 1.7 Results of evaluation X X X X X X XX X X

[0208] As is apparent from Tables D1 to D4, for steel products producedusing short LF, long RH treatment according to the present inventionwherein a molten steel produced in an arc melting furnace or a converteris transferred to a ladle furnace to perform ladle refining for a shortperiod of time, i.e., not more than about 60 min, and is then circulatedthrough a circulation-type vacuum degassing device to increase the RHcirculation quantity (that is, amount of molten metal circulated/totalamount of molten metal) and to perform degassing for a long period oftime, i.e., not less than 25 min and for steels producing using acombination of short LF, long RH treatment+high-temperature tapping at atemperature above the conventional operation, i.e., melting point+atleast 100° C., for both steel types, SUJ 2 and SCM 435, the oxygencontent of the products is small and, in addition, the number ofinclusions having a size of not less than 20 μm is significantlydecreased. As can be seen from Tables D1 to D4, for the examples of thepresent invention, all the steel products are evaluated as good (◯) orexcellent (⊚), that is, are excellent high-cleanliness steels. Bycontrast, as can be seen from Tables D5 and D6, for all the conventionalexamples, the cleanliness is evaluated as failure (X), and theconventional steel products cannot be said to be clean steels.

[0209] For the heats wherein a molten steel is subjected to oxidizingμmelting in an arc melting furnace or a converter, both the oxygencontent, and the predicted value of the maximum inclusion diameter arereduced by increasing T_(SH) [(temperature at which molten steel istransferred to ladle furnace)−(melting point of molten steel)=T_(SH))]to improve the cleanliness. For the heats, regarding the relationship ofthe refining time in the ladle furnace with the oxygen content and thepredicted value of the maximum inclusion diameter, when the refiningtime is not more than 60 min, for example, is short and about 25 min,the oxygen content and the predicted value of the maximum inclusiondiameter are satisfactorily lowered. The predicted value of the maximuminclusion diameter, however, increases with increasing the refiningtime. The reason for this is considered as follows. With the elapse oftime, the melt loss of refractories in the ladle furnace is increased,the equilibrium of the slag system is broken, for example, as a resultof oxidation due to the contact with the air, and the level of thedissolved oxygen goes beyond the minimum level of dissolved oxygen.Further, the relationship of the amount of molten steel circulated/totalamount of molten steel in the circulation-type vacuum degassing devicewith the oxygen content and the predicted value of the maximum inclusiondiameter, the effect of enhancing the cleanliness increases withincreasing the amount of molten steel circulated, that is, withincreasing the degassing time, and is substantially saturated when theamount of molten steel circulated/total amount of molten steel is notless than 15 times.

[0210] It was confirmed that reducing the oxygen content and thepredicted value of the maximum inclusion diameter results in improvedL₁₀ life. This indicates that steels produced by the process accordingto the present invention, which can reduce the oxygen content and thepredicted value of the maximum inclusion diameter, have excellentfatigue strength properties such as excellent rolling fatigue life.

[0211] FIG. D1 is a diagram showing the oxygen content of products in 10heats in the production process according to the present inventionwherein, in the treatment of a molten steel for steel SUJ 2, a moltensteel, which had been subjected to oxidizing refining and produced by amelting process in an arc melting furnace or a converter, is transferredto a ladle furnace to perform ladle refining for a short period of timeand is then subjected to circulation-type vacuum degassing for a longperiod of time, and the oxygen content of products in 10 heats in theconventional process wherein a molten steel, which had been subjected tooxidizing refining and produced by a melting process in an arc meltingfurnace or a converter, is transferred to a ladle furnace to performladle refining for a long period of time and is then subjected tocirculation-type vacuum degassing for a short period of time. In FIGS.D1, D3, and D5, A₁ shows data on the adoption of short-time LF,long-time RH treatment according to the present invention defined inclaim 22, A₂ data on the adoption of a combination of high-temperaturetapping+short-time LF, long-time RH treatment according to the presentinvention defined in claim 23, and conventional data on the conventionalprocess.

[0212] FIG. D2 is a diagram showing the oxygen content of products in 10heats in the production process according to the present inventionwherein, in the treatment of a molten steel for steel SCM 435, a moltensteel, which had been subjected to oxidizing refining and produced by amelting process in an arc melting furnace or a converter, is transferredto a ladle furnace to perform ladle refining for a short period of timeand is then subjected to circulation-type vacuum degassing for a longperiod of time, and the oxygen content of products in 10 heats in theconventional process wherein a molten steel, which had been subjected tooxidizing refining and produced by a melting process in an arc meltingfurnace or a converter, is transferred to a ladle furnace to performladle refining for a long period of time and is then subjected tocirculation-type vacuum degassing for a short period of time. In FIGS.D1, D3, and D5, A₁ shows data on the adoption of short-time LF,long-time RH treatment according to the present invention defined inclaim 22, A₂ data on the adoption of a combination of high-temperaturetapping+short-time LF, long-time RH treatment according to the presentinvention defined in claim 23, and conventional data on the conventionalprocess.

[0213] FIG. D3 is a diagram showing the maximum predicted inclusiondiameter determined according to statistics of extreme values inproducts in 10 heats in the production process according to the presentinvention wherein, in the treatment of a molten steel for steel SUJ 2,the process according to claim 22 or 23 of the present invention iscarried out, and the maximum predicted inclusion diameter determinedaccording to statistics of extreme values in products in 10 heats in theconventional process wherein, in the treatment of a molten steel forsteel SUJ 2, long-time LF, short-time RH treatment is carried out.

[0214] FIG. D4 is a diagram showing the maximum predicted inclusiondiameter determined according to statistics of extreme values inproducts in 10 heats in the production process according to the presentinvention wherein, in the treatment of a molten steel for steel SCM 435,the process according to claim 22 or 23 of the present invention iscarried out, and the maximum predicted inclusion diameter determinedaccording to statistics of extreme values in products in 10 heats in theconventional process wherein, in the treatment of a molten steel forsteel SCM 435, long-time LF, short-time RH treatment is carried out.

[0215] FIG. D5 shows data on L₁₀ life as determined by a thrust rollingservice life test in products in 10 heats in the production processaccording to the present invention wherein, in the treatment of a moltensteel for steel SUJ 2, the process according to claim 22 or 23 of thepresent invention is carried out, and the L₁₀ life as determined by thethrust rolling service life test in products in 10 heats in theconventional process wherein, in the treatment of a molten steel forsteel SUJ 2, long-time LF, short-time RH treatment is carried out.

[0216] FIG. D6 shows data on L₁₀ life as determined by a thrust rollingservice life test in products in 10 heats in the production processaccording to the present invention wherein, in the treatment of a moltensteel for steel SCM 435, the process according to claim 22 or 23 of thepresent invention is carried out, the L₁₀ life as determined by thethrust rolling service life test in products in 10 heats in theconventional process wherein, in the treatment of a molten steel forsteel SCM 435, long-time LF, short-time RH treatment is carried out.

[0217] As is apparent from the test results, it was confirmed that, forboth steel SUJ 2 and steel SCM 435, the process, in which a moltensteel, which had been subjected to oxidizing refining and produced by amelting process in an arc melting furnace or a converter, is transferredto a ladle furnace to perform ladle refining for a short period of timeand is then circulated through a circulation-type vacuum degassingdevice to perform degassing for a long period of time, can significantlyreduce the oxygen content of the products, and the predicted value ofthe maximum inclusion diameter and, according to the process of thepresent invention, the cleanliness is significantly improved and the L₁₀life as determined by the thrust rolling service life test issignificantly improved. The addition of treatments to the process, thatis, the addition of short-time LF, long-time RH treatment according tothe present invention defined in claim 22, and the addition ofhigh-temperature tapping+short-time LF, long-time RH treatment accordingto the present invention defined in claim 23, can significantly improveall the oxygen content of products, the predicted value of the maximuminclusion diameter, and the L₁₀ life as determined by the thrust rollingservice life test.

[0218] As is apparent from the foregoing description, the presentinvention can provide a large quantity of steel products having a veryhigh level of cleanliness without use of a remelting process whichincurs very high cost. This can realize the provision ofhigh-cleanliness steels for use as steels for mechanical parts requiredto possess fatigue strength, fatigue life, and quietness, particularly,for example, as steels for rolling bearings, steels for constantvelocity joints, steels for gears, steels for continuously variabletransmission of toroidal type, steels for mechanical structures for coldforging, tool steels, and spring steels, and processes for producing thesame, that is, can offer unprecedented excellent effect.

EXAMPLE E

[0219] A molten steel of JIS SCM 435, which had been subjected tooxidizing refining and produced by a melt process in an arc furnace, wastransferred to a ladle furnace provided with an electromagneticinduction stirrer where 50 to 80 min in total of ladle refining(stirring by gas for a short time in an inert atmosphere+electromagneticstirring) was carried out. Next, degassing was carried out for 20 to 30min. In particular, degassing was carried out in a circulation-typedegassing device in such a manner that the amount of the molten steelcirculated was not less than 12 times the total amount of the moltensteel, followed by an ingot production process using casting to producesteel products of SCM 435 in 10 heats. For comparison, a molten steel ofJIS SCM 435, which had been subjected to oxidizing refining and producedby a melt process in the same manner as described above in an arcfurnace through the conventional operation, was transferred to a ladlefurnace where the molten steel was stirred by gas for 35 to 50 min toperform ladle refining. Next, circulation-type degassing was carried outfor not more than 25 min, followed by an ingot production process usingcasting to produce steel products of SCM 435 in 10 heats. These productsthus obtained were examined for the oxygen content of the products, thepredicted value of the maximum inclusion diameter according tostatistics of extreme values, and L₁₀ service life by a thrust-typerolling service life test. In the measurement of the predicted value ofthe maximum inclusion diameter, a test piece was taken off from a φ65forged material, the observation of 100 mm² was carried out for 30 testpieces, and the maximum inclusion diameter in 30000 mm² was predictedaccording to statistics of extreme values. In the thrust-type rollingservice life test, a test piece having a size of φ60×φ20×8.3T, which hadbeen subjected to carburizing, quench hardening and tempering, wastested at a maximum hertz stress Pmax: 4900 MPa, followed by calculationto determine the L₁₀ service life.

[0220] An example of the operation of the present invention and testresults are shown in Table E1, and a comparative example of theconventional operation and test results are shown in Table E2. TABLE E1Out-furnace (ladle) refining by (short-time stirring by gas +electromagnetic Operation stirring) No. 1 2 3 4 5 6 7 8 9 10 Type ofsteel SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435SCM 435 SCM 435 Out-furnace refining furnace: 55 76 70 78 59 65 68 53 6977 Time, min Out-furnace refining furnace: 1577 1581 1577 1585 1584 15821582 1576 1582 1584 Termination temp., ° C. RH: Time, min 28 21 24 22 2128 26 25 25 28 RH: Quantity of circulation, 9.3 7.0 8.0 7.3 7.0 9.3 8.78.3 8.3 9.3 times RH: Termination temp., ° C. 1534 1540 1534 1540 15411532 1539 1531 1538 1532 Casting temp., ° C. 1512 1513 1516 1513 15131515 1512 1516 1514 1518 Oxygen content of product, 6.3 5.5 5.5 5.4 6.06.0 6.6 6.5 5.7 5.6 ppm Number of inclusions of not 13 6 11 9 5 8 11 1410 14 less than 20 μm in 100 g of steel product Maximum predicteddiameter 30.2 25.3 26.4 24.3 28.8 27.0 26.9 30.6 26.2 25.8 ofinclusions, μm L₁₀ (× 10⁷) 9.2 10.0 8.4 8.9 11.3 10.7 10.8 9.4 9.8 9.3Results of evaluation ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0221] TABLE E2 Operation Out-furnace (ladle) refining by short-timestirring by gas No. 1 2 3 4 5 6 7 8 9 10 Type of steel SCM 435 SCM 435SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435 SCM 435Out-furnace refining furnace: 35 45 48 38 42 47 42 39 48 44 Time, minOut-furnace refining furnace: 1570 1574 1566 1572 1567 1569 1567 15691569 1570 Termination temp., ° C. RH: Time, min 24 23 21 23 23 23 23 2321 23 RH: Quantity of circulation, 6.7 7.5 6.2 7.3 7.0 6.8 6.0 8.0 7.48.3 times RH: Termination temp., ° C. 1531 1538 1541 1531 1541 1533 15331534 1535 1540 Casting temp., ° C. 1521 1519 1517 1512 1520 1517 15111512 1516 1518 Oxygen content of product, 9.1 9.2 6.9 9.4 9.2 7.6 8.38.3 6.9 8.8 ppm Number of inclusions of not 42 54 42 53 59 49 57 53 5652 less than 20 μm in 100 g of steel product Maximum predicted diameter91.0 82.8 55.2 84.6 73.6 68.4 83.0 83.0 55.2 70.4 of inclusions, μm L₁₀(× 10⁷) 2.0 1.7 2.6 2.1 1.0 1.1 1.8 1.4 2.2 1.7 Results of evaluation XX X X X X X X X X

[0222] As is apparent from Table E1, for SCM 435 steel products of 10heats produced according to the process of the present invention,wherein a molten steel of JIS SCM 435, which has been subjected tooxidizing refining and produced by a melt process in an arc furnace, istransferred to a ladle furnace provided with an electromagneticinduction stirrer, where 50 to 80 min in total of ladle refining(stirring by gas for a short time in an inert atmosphere+electromagneticstirring) is carried out, and the molten steel is degassed for 20 to 30min, in particular, degassing is carried out in a circulation-typedegassing device in such a manner that the amount of the molten steelcirculated is not less than 12 times the total amount of the moltensteel, followed by an ingot production process using casting, that is,steel Nos. 1 to 10, the oxygen content of the product is 5.4 to 6.6 ppm,the number of inclusions having a size of not less than 20 μm per 100 gof the steel product is 5 to 14, and the maximum predicted inclusiondiameter is 30.6 μm. That is, these products are very clean steels.Further, these products have very highly improved L₁₀ life. For theoverall evaluation, all of these products are evaluated as very good(⊚).

[0223] By contrast, as can be seen in Table E2, for SCM 435 steelproducts of 10 heats produced according to the comparative conventionalprocess, wherein a molten steel of JIS SCM 435, which has been subjectedto oxidizing refining and produced by a melt process in an arc furnace,is transferred to a ladle furnace where the molten steel is stirred bygas for 35 to 50 min to perform ladle refining, and the molten steel issubjected to circulation-type degassing for not more than 25 min,followed by an ingot production process using casting, the oxygencontent of the product is slightly larger than that in the presentinvention although the oxygen content is relatively low. Further, thenumber of inclusions having a size of not less than 20 μm per 100 g ofthe steel product is much larger than that in the present invention andis 42 to 59, and the maximum predicted inclusion diameter is also largerthan that in the present invention and is 55.2 to 91.0 μm. Further, theL₁₀ life is also lower than that in the present invention and isone-tenth to one-fifth of that in the present invention. All thecomparative steels are evaluated as failure (X).

[0224] The above examples demonstrate that the process according to thepresent invention can lower the oxygen content and the predicted valueof the maximum inclusion diameter, and the L₁₀ life is improved. Thisindicates that steels produced according to the process of the presentinvention, which can reduce the oxygen content and the predicted valueof the maximum inclusion diameter, have excellent fatigue strengthproperties, such as excellent rolling fatigue service life.

[0225] As is apparent from the foregoing description, the presentinvention can provide a large quantity of steel products having a veryhigh level of cleanliness without use of a remelting process whichincurs very high cost. This can realize the provision ofhigh-cleanliness steels for use as steels for mechanical parts requiredto possess fatigue strength, fatigue life, and quietness, particularly,for example, as steels for rolling bearings, steels for constantvelocity joints, steels for gears, steels for continuously variabletransmission of troidal type, steels for mechanical structures for coldforging, tool steels, and spring steels, and processes for producing thesame, that is, can offer unprecedented excellent effect.

1. A process for producing a high-cleanliness steel, comprising thesteps of: transferring a molten steel produced in an arc melting furnaceor a converter to a ladle furnace to refine the molten steel; degassingthe molten steel; and then casting the molten steel into an ingot, saidprocess further comprising the step of tapping deoxidation wherein, intransferring the molten steel to the ladle furnace, a deoxidizerincluding manganese, aluminum, and silicon is added to the molten steelby previously placing the deoxidizer in the ladle, and/or by adding thedeoxidizer to the molten steel in the course of tapping into the ladle,whereby the molten steel is pre-deoxidized before the refining in theladle furnace.
 2. The process according to claim 1, wherein the moltensteel is transferred to the ladle furnace in such a manner that thetapping temperature of the molten steel is at least 100° C. above themelting point of the steel.
 3. The process according to claim 1, whereinthe refining in the ladle furnace is carried out for not more than 60min, and the degassing is carried out for not less than 25 min.
 4. Ahigh-cleanliness steel produced by the process according to any one ofclaims 1 to
 3. 5. The high-cleanliness steel according to claim 4,wherein the content of oxygen in the steel is not more than 10 ppm. 6.The high-cleanliness steel according to claim 4, wherein the number ofoxide inclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid is not more than 40 per 100 g ofthe steel product.
 7. The high-cleanliness steel according to claim 4,wherein the predicted value of the maximum inclusion diameter in 30000mm² as calculated according to statistics of extreme values is not morethan 60 μm.
 8. A process for producing a high-cleanliness steel,comprising the steps of: degassing a molten steel produced in an arcmelting furnace or a converter; transferring the degassed molten steelto a ladle furnace to refine the molten steel; and circulating therefined molten steel through a circulation-type vacuum degassing deviceto further degas the molten steel.
 9. The process according to claim 8,wherein the molten steel is transferred to the ladle furnace in such amanner that the tapping temperature of the molten steel is at least 100°C. above the melting point of the steel.
 10. The process according toclaim 8, wherein the refining in the ladle furnace is carried out fornot more than 60 min, and the degassing subsequent to the ladle refiningis carried out for not less than 25 min.
 11. A high-cleanliness steelproduced by the process according to any one of claims 8 to
 10. 12. Thehigh-cleanliness steel according to claim 11, wherein the content ofoxygen in the steel is not more than 10 ppm.
 13. The high-cleanlinesssteel according to claim 11, wherein the number of oxide inclusionshaving a size of not less than 20 μm as detected by dissolving the steelproduct in an acid is not more than 40 per 100 g of the steel product.14. The high-cleanliness steel according to claim 11, wherein thepredicted value of the maximum inclusion diameter in 30000 mm² ascalculated according to statistics of extreme values is not more than 60μm.
 15. A process for producing a high-cleanliness steel, comprising thesteps of: subjecting a molten steel to oxidizing refining in an arcmelting furnace or a converter; adding a deoxidizer to the molten steelin the furnace before tapping to deoxidize the molten steel;transferring the deoxidized molten steel to a ladle furnace to performladle refining; and then circulating the refined molten steel through acirculation-type vacuum degassing device to degas the molten steel. 16.The process according to claim 15, wherein the molten steel istransferred to the ladle furnace in such a manner that the temperatureof the molten steel to be transferred is at least 100° C. above themelting point of the steel.
 17. The process according to claim 15,wherein the refining in the ladle furnace is carried out for not morethan 60 min, and the degassing in the circulation-type vacuum degassingdevice is carried out for not less than 25 min.
 18. A high-cleanlinesssteel produced by the process according to any one of claims 15 to 17.19. The high-cleanliness steel according to claim 18, wherein thecontent of oxygen in the steel is not more than 10 ppm.
 20. Thehigh-cleanliness steel according to claim 18, wherein the number ofoxide inclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid is not more than 40 per 100 g ofthe steel product.
 21. The high-cleanliness steel according to claim 18,wherein the predicted value of the maximum inclusion diameter in 30000mm² as calculated according to statistics of extreme values is not morethan 60 μm.
 22. A process for producing a high-cleanliness steel,comprising the steps of: transferring a molten steel produced in an arcmelting furnace or a converter to a ladle refining furnace to refine themolten steel; subjecting the refined molten steel to circulation-typevacuum degassing; and then casting the degassed molten steel into aningot, wherein the refining in the ladle furnace is carried out for notmore than 60 min, and the degassing in the circulation-type vacuumdegassing device is carried out for not less than 25 min under suchconditions that the amount of the molten steel circulated in thecirculation-type vacuum degassing device is at least 8 times larger thanthe total amount of the molten steel.
 23. The process according to claim22, wherein the molten steel is transferred to the ladle furnace in sucha manner that the temperature of the molten steel to be transferred isat least 100° C. above the melting point of the steel.
 24. Ahigh-cleanliness steel produced by the process according to claim 22 or23.
 25. The high-cleanliness steel according to claim 24, wherein thecontent of oxygen in the steel is not more than 10 ppm.
 26. Thehigh-cleanliness steel according to claim 24, wherein the number ofoxide inclusions having a size of not less than 20 μm as detected bydissolving the steel product in an acid is not more than 40 per 100 g ofthe steel product.
 27. The high-cleanliness steel according to claim 24,wherein the predicted value of the maximum inclusion diameter in 30000mm² as calculated according to statistics of extreme values is not morethan 60 μm.
 28. A process for producing a high-cleanliness steel,comprising the steps of: transferring a molten steel produced in an arcmelting furnace or a converter to a ladle where the molten steel isrefined by gas stirring; subjecting the molten steel to circulation-typevacuum degassing; and then casting the degassed molten steel into aningot, wherein an electromagnetic induction stirrer is provided in theladle and, in addition to the gas stirring, electromagnetic stirring iscarried out for 50 to 80 min, thereby performing ladle refining.
 29. Theprocess according to claim 28, wherein the ladle refining by the gasstirring and the electromagnetic stirring in the ladle is carried out inan inert atmosphere.
 30. A high-cleanliness steel produced by theprocess according to claim 28 or
 29. 31. The high-cleanliness steelaccording to claim 30, wherein the content of oxygen in the steel is notmore than 10 ppm.
 32. The high-cleanliness steel according to claim 30,wherein the number of oxide inclusions having a size of not less than 20μm as detected by dissolving the steel product in an acid is not morethan 40 per 100 g of the steel product.
 33. The high-cleanliness steelaccording to claim 30, wherein the predicted value of the maximuminclusion diameter in 30000 mm² as calculated according to statistics ofextreme values is not more than 60 μm.